Our lives are full of instances where have to hold ourselves back. We stop ourselves from eating that tempting slice of cake to avoid putting on weight. We bite our tongues to avoid insulting our friends. We slam on the brakes to avoid killing a pedestrian.  To quote Yoda: “Control! Control! You must learn control.”

People with drug problems clearly have a problem with this. Their ability to resist their own impulses falters at the promise of the next hit. Now, scientists are starting to understand the changes in the brain that underlie these problems.

Karen Ersche from the University of Cambridge found that drug users have abnormalities in parts of the brain that are important for inhibiting unwanted actions. These same anomalies even exist in the brains of their siblings, who don’t have any drug problems themselves. They could act as a marker for people who are vulnerable to addiction. “Our findings provide further evidence for drug addiction being a brain-based disorder,” says Ersche.

This is far from the first study to examine the brains of drug users. But it’s never been clear whether changes in such brains were caused by drugs, or made people vulnerable to addiction in the first place. Both are possible. Stimulant drugs typically act on parts of the brain involved in motivation, and interfere with those that inhibit our impulses. But these effects could be worse if these neural circuits are already weak.

To separate these possibilities, Ersche studied 50 volunteers who had a long history of drug abuse. She compared them to their siblings, who had no drug problems, and to 50 unrelated volunteers who were also drugs-free. All of the recruits sat through a stop-signal test – a commonly used way of measuring self-control. Volunteers have to respond as quickly as possible to a stream of on-screen symbols – say, by pressing a key. If they hear a tone, which pops up unpredictably, they have to restrain themselves. (Try it yourself here).

The drug users struggled with the test compared to the unrelated volunteers, and needed more time to withhold their responses. Critically, their siblings fared just as badly, even though they weren’t using drugs. This strongly suggests that poor self-control isn’t the result of the drugs themselves, but of a shared (and probably inherited) vulnerability. “If you have brain with existing problems, the drugs have an easier play. It’s easier for them to take over,” says Ersche.

Ersche found the same pattern when she looked at her volunteers’ brains. First, she focused on their white matter tracts – the fibres that transmit signals from one area to another. These are the brain’s communications network, and their density indicates how good different areas are at shuttling information between them.

These connections were weaker among both the drug users and their relatives, compared to the healthy unrelated volunteers. The fibres were particularly sparse around the right inferior frontal cortex (IFC), an area involved in controlling our inhibitions. These abnormalities were linked to the volunteers’s scores on the stop-signal test – the weaker the connections, the slower their reaction times. With its communication lines weakened, the IFC was less able to exert its suppressive influence.

The siblings also shared anomalies in the size of some brain areas. Their putamens and medial temporal lobes were bigger, and their posterior insulas were smaller. All of these areas have been implicated in learning and memory. “This may be an indicator of an enhanced propensity to form habits,” says Ersche.

From these results, a cohesive picture emerges. Some parts of the brain are larger, increasing the attractiveness of potential rewards, and the odds of habitual, addictive behaviour. The IFC, which would normally suppress such desires, has less of a say because the fibres connecting it to other parts of the brain are weaker. It’s like having a mob of reckless friends who are egging each other on over fast broadband connections, while their sensible parents send them words of caution on a dial-up modem.

This is uncannily similar to what happens in  the teenage brain, where areas associated with reward mature before the prefrontal areas that exercise restraint. Other scientists have suggested that this gap in timing explains why teens are so prone to risky and impulsive behaviours. They’re not making thoughtless decisions – they simply weigh risks and rewards in a different way to adults. Perhaps people who are vulnerable to addiction never grow out of this asymmetry between desire and inhibition. “It does look like a developmental problem,” says Ersche, “but we really need to compare these brains to those of adolescents to know for sure.”

“This is a very important and well-designed study,” says Susan Tapert from the University of California, San Diego. She adds, “It will be important to understand how the non-drug dependent volunteers were able to avoid drug problems given same brain features as their siblings.”

This is a key point. Drug dependence runs in families, and it is clearly influenced by a person’s genes. But genes do not determine behaviour; they merely influence it. The non-addicted siblings in Ersche’s study illustrate the point beautifully. “They share so much,” says Ersche. “They have the same vulnerabilities as their drug-dependent brothers and sisters. They had a lot of domestic violence and troubled childhoods but they didn’t get into drugs. Their average age was 33. They may have had many opportunities to develop dependence, but they didn’t.”

Perhaps the other one had environmental influences that set them down a different path. Perhaps they also had inherited some “resilience factors” that their siblings did not.  In an earlier study with some of the same siblings, Ersche found that all of them are more impulsive, but only the drug users were “sensation-seekers”. These are subtly different traits. “Impulsive people act on the spur of the moment,” Ersche explains, “but sensation-seekers crave excitement and adventure. In contrast to the drug-dependent individuals, their siblings do not seem to crave for excitement and sensations, which might have protected them from taking drugs in the first place.

In the meantime, Ersche’s study suggests that the white fibre tracts around the IFC could be used as a marker for vulnerability to addiction. That’s useful for two reasons. We could use it to identify people who are most at risk of abusing drugs, before they actually encounter any problems. We could also see if people can strengthen the connections in this critical area. Many scientists have developed programmes for improving self-control at an early age. Monitoring the IFC’s white matter could provide an objective way of measuring whether those programmes are working. As Tapert says, “We might be able to modify these risky brain characteristics, to see if the misuse of drugs can be reduced.”

Reference: Ersche, Jones, Williams, Turton, Robbins & Bullmore. 2011. Abnormal Brain Structure Implicated in Stimulant Drug Addiction. Science http://dx.doi.org/10.1126/science.1214463

Eventually, it was the psychologist Oskar Pfungst who debunked Hans’ extraordinary abilities. He showed that the horse was actually responding to the expectations of its human interrogators, reading subtle aspects of their posture and expressions to work out when it had tapped enough. The legend of Hans’ intellect was consigned to history. But history, as we know, has a habit of repeating itself.

For the last few decades, psychologists have been using a technique called priming. With subtle hints of words or concepts, they can trigger impressive changes in behaviour. Words of cleanliness can make people behave more morally. Words related to age can slow their bodies. Words of power sharpen our mental abilities. All of these studies have suggested that our behaviour is influenced by subtle things that lie beneath the watch of our conscious awareness.

This view could well be right, but not always in the way that psychologists believe. Stephane Doyen from the Université Libre de Bruxelles has repeated one of the classic experiments in priming and shown that, in this case at least, it’s not the words that create the effect. It’s the experimenters’ expectations.

Back in 1996, John Bargh and his colleagues found that infusing people’s minds with the concept of age could slow their movements (PDF). The volunteers in the study had to pick the odd word from a group of scrambled ones. When this word related to being old, the volunteers walked more slowly when they left the laboratory. They apparently didn’t notice anything untoward about the words, but their behaviour changed nonetheless. It was a classic result, and it turned the paper into one of the most widely cited studies in social psychology.

Two other groups have since replicated the effect, but neither stuck to the original set-up. That’s what Doyen wanted to do, but with three important tweaks. First, in Bargh’s study, a researcher timed the volunteers with a stopwatch. This time, Doyen would use infrared sensors for more accurate readings. Second, Bargh recruited 60 volunteers, but Doyen recruited twice as many. Third, Doyen also recruited four experimenters who carried out the study, but didn’t know what the point of it was.

This time, the priming words had no impact on the volunteers’ walking speed. They left the test room neither more slowly nor more quickly than when they arrived. The famous result hadn’t replicated. Why?

Doyen suspected that Bargh’s research team could have unwittingly told their volunteers how they were meant to behave, just as von Osten unconsciously told Clever Hans when to stamp. Perhaps they themselves moved more slowly if they expected the volunteer to do so. Maybe they spoke more languidly, or shook hands more leisurely.

We know that this sort of thing goes on all the time. It’s why people who run medical trials use “double-blind” designs, where neither experimenters nor patients know who is being given what. That wasn’t the case in Bargh’s study. The experimenter who clocked the stopwatch in the corridor didn’t know which volunteers had been primed, but the experimenter in the test room did. They could have unconsciously amplified the effect of their primes. Maybe they were responsible for creating the very behaviour they expected to see.

To test that idea, Doyen repeated his experiment with 50 fresh volunteers and 10 fresh experimenters. The experimenters always stuck to the same script, but they knew whether each volunteer had been primed or not. Doyen told half of them that people would walk more slowly thanks to the power of priming, but he told the other half to expect faster walks. Again, he measured the volunteers’ speed with infrared sensors, but he also gave the experiments a stopwatch to take some back-up readings.

When Doyen looked at the data from the infrared sensors, he found that the volunteers moved more slowly only when they were tested by experimenters who expected them to move slowly. If Doyen relied on the experimenters’ own stopwatch-based measurements, things were even worse. The ones who anticipated faster walks measured faster walks. The ones that presumed slower walks found those too. Let that sink in: the only way Doyen could repeat Bargh’s results was to deliberately tell the experimenters to expect those results.

It’s a fascinating result, but one that isn’t a deathblow for priming as a method. Note that Doyen isn’t suggesting that Bargh’s team were simply making up their results to fit what they expected. Rather, their expectations affected their behaviour, which then affected the volunteers’ behaviour. The volunteers were still being primed, albeit by the experimenters rather than the word tasks. “Either possibility is a confirmation for the power of priming,” says Tom Stafford from the University of Sheffield.

Bargh himself says, “The basic ‘stereotype-priming of behavior’ effect has been replicated dozens of times. There are many reasons for a study not to work, and as I had no control over [this] attempt, there’s not much I can say.”

Joshua Ackerman, a psychologist from MIT, says, “There have been hundreds, if not thousands, of priming studies conducted, many of which use designs that make experimenter bias essentially impossibl. It would be a huge mistake to draw the implication here that [these] studies refute this body of work in any way.”

Doyen’s study doesn’t show a radically new flaw, or one that’s unique to this branch of social psychology. We’ve known for over a century that scientists can very easily bias their own experiments, even in the most carefully controlled cases. “It’s a neat paper that re-emphasises some highly important and widely relevant warnings for everyone who might want to conduct experiments with people,” says Stafford. “Expectations – participants’ and experimenters’ – and inaccurate measurement can combine to give you biased results.” Ackerman adds, “It’s a lesson that behavioural researchers are all trained in, but one that bears repeating from time to time.”

“Our results don’t completely rule out the possibility of unconscious priming,” says Doyen, “but they point to the fact that the (generally weak) effects may also be influenced by many other factors that are almost never controlled in such studies.”

The study also serves as a good reminder about how important it is for scientists to try and repeat each others’ results. “The need for independent replications of important results such as those of Bargh cannot be overstated,” says Doyen. “The literature relies far too much on findings that have been produced using different methods dating back to 30 or 40 years ago.”

Reference: Doyen, Klein, Pichon & Cleeremans. 2011. Behavioral Priming: It’s all in the Mind, but Whose Mind? PLoS ONE http://dx.doi.org/10.1371/journal.pone.0029081

Image by Alex Proimos

But Claudia Fritz (a scientist who studies instrument acoustics) and Joseph Curtin (a violin-maker) may have discovered the real secret to a Stradivarius’s sound: nothing at all.

The duo asked professional violinists to play new violins, and old ones by Stradivari and Guarneri. They couldn’t tell the difference between the two groups. One of the new violins even emerged as the most commonly preferred instrument.

Ever since the early 19^th century, many tests have questioned the alleged superiority of the old Italian violins. Time and again, listeners have failed to distinguish between the sound of the old and new instruments. But critics have been quick to pick holes in these studies. In most cases, the listeners weren’t experts, and the players and researchers knew which violin was which – a flaw that could have biased the results.

What’s more, no one has tested whether violinists themselves can truly pick up the supposedly distinctive sound of a Strad. The common wisdom is that they can, but Fritz and Curtin showed that this isn’t true. “Many people were convinced that as soon as you play an old violin, you can feel that it’s old, it’s been played a lot, and it has a special sound quality,” says Fritz. “People who took part in the experiment said it was the experience of a lifetime when we told them the results. They were fully convinced they could tell the difference, and they couldn’t.”

During the Eighth International Violin Competition of Indianapolis – one of the world’s most important competitions – Fritz and Curtin persuaded six violinists to part with their instruments. Three of the violins were new; one was made a few days before. The other three had illustrious, centuries-long histories. Two were made by Stradivari and the other by Guarneri. One of the Stradivari, denoted “O1”, currently belongs to an institution, and is loaned to only the most gifted players. All three have featured in concerts and recordings, bowed by famous violinists. Their combined value is around 10 million US dollars, a hundred times more than the three new ones.

Curtin’s influence was essential in persuading people to give up such prized, fragile possessions, especially to be played by blindfolded strangers. “Joseph is a well-known person in the community and people trust him,” says Fritz. “That’s why we managed to do the study: the combination of me as the scientist and him as the violin-maker.”

Back in the lab, Fritz and Curtin asked 21 professional volunteers to play the six violins. They had played for anywhere from 15 to 61 years, and some of them were even involved in the competition as contestants and judges. They played the instruments in a dimly lit hotel room chosen for relatively dry acoustics.

The test was a true “double-blind” one, as neither the players nor the people who gave them the violins had any way of knowing which instrument was which. The room was dimly lit. The players were wearing goggles so they couldn’t see properly. The instruments had dabs of perfume on the chinrests that blocked out any distinctive smells. And even though Fritz and Curtin knew which the identities of the six violins, they only passed the instruments to the players via other researchers, who were hidden by screens, wearing their own goggles, and quite literally in the dark.

First, the players were given random pairs of violins. They played each instrument for a minute, and said which they preferred. Unbeknownst to them, each pair contained an old violin and a new one. For the most part, there was nothing to separate the two, and the players preferred the new instrument as often as the old one. There was one exception: O1, the Stradivarius with the most illustrious history, was chosen far less often than any of the three new violins.

Next, Fritz and Curtin gave the recruits a more natural task. They saw all six violins, laid out in random order on a bed. They had 20 minutes to play any violin against any other and to choose the one they’d most like to take home. They also picked the best and worst instruments in terms of four qualities: range of tone colours; projection; playability; and response.

This time, a clear favourite emerged. The players chose one of the new violins (“N2”) as their take-home instrument most often, and it topped the rankings for all four qualities. As before, O1 received the most severe rejections. Overall, just 38 percent of the players (8 out of 21) chose to take an old violin home, and most couldn’t tell if their instrument was old or new. As Fritz and Curtin write, this “stands as a bracing counterexample to conventional wisdom.”

There are some issues with the study. Curtin, being a maker of new violins, has an obvious bias, but the double-blind design should have prevented that from affecting the results. The sample size – six violins and 21 players – is fairly small, but as large as can be expected when dealing with rare and incredibly expensive objects. There might also other variables that could affect the players’ perceptions – perhaps, for example, they might feel differently in rooms with different acoustics.

Fritz expects scepticism. She says, “It might help to change people’s mentality, but quite slowly. It’s a very conservative community. We’ll probably get critics saying we didn’t take this or that into account, but obviously, it was the same for the new violins too.” She adds, “Modern makers should be very happy, and we hope that it’ll help them to promote their violins. It shows that they’re doing a great job and their violins are on a par with the old ones.”

Perhaps the esteem that’s placed on Stradivarius violins is less about the triumph to age-old craftsmanship, and more a testament to our ability to delude ourselves. This ability has come out in other areas. Take wine, another product where certain specimens fetch critical acclaim and exorbitant prices on the basis of superior quality. And yet, study after study has shown that expensive wines taste the same as cheap plonk when you test people under double-blind conditions. The imagined link between price and quality is a delusion but, as Jonah Lehrer skilfully argues, it can be a pleasant one.

The same could be said of violins. The joy of owning and playing a Stradivarius comes not from any objective advantage in its sound, but simply from the knowledge that it is a Stradivarius. Never mind what it sounds like – it’s an elegant and beautifully made instrument that carries status in its name, gravitas in its price tag, and the weight of centuries in its wood.

For this reason, studies like this are useful for busting some myths, and they may boost the credibility of new violins, but they are unlikely to diminish the lust for the old ones. Fritz and Curtin recognise as much. Writing about one of their volunteers, they say, “When asked the making-school of the new instrument he had just chosen to take home, he smiled and said only, “I hope it’s an [old] Italian.”

UPDATE: John Soloninka, one of the 21 violinists who took part in the study, has commented about his experiences below: “It was fascinating. I too, expected to be able to tell the difference, but could not. Claudia sent me my comments about the instruments that I made while I was playing them, and it was hilarious how wrong my impressions were at the time!”

Reference: Fritz, Curtin, Poitevineau, Morrel-Samuels & Tao. 2011. Player preferences among new and old violins. PNAS http://dx.doi.org/10.1073/pnas.1114999109

Image: by Håkan Svensson

Among animals too, the underdogs often become the victors. One such example exists in the rainforests of Panama. There, capuchin monkeys live in large groups, each with its own territory. The monkeys often invade each other’s land. Numbers provide an obvious advantage in such conflicts, but small groups can often successfully defend their territory against big ones. Unlike human underdogs, they don’t win because of superior tactics or weapons. They win because their rivals are full of deserters.

Margaret Crofoot from the Smithsonian Tropical Research Institute, and Ian Gilby from Duke University have spent many years studying wild white-faced capuchins on Barro Colorado Island in Panama. They’ve fitted individuals from six different groups with radio collars, so they can be easily tracked.

In 2008, they showed that on the whole, larger groups are more likely to win fights than smaller ones. Each extra member increases the odds of victory by 10 percent. But these odds vary greatly depending on where the capuchins are. For every 100 metres that they move away from the centre of their territory, their odds of victory fall by 31 percent. If they invade another group’s patch, their distance from home can greatly outweigh the benefits of a large party.

Now, Crofoot and Gilby have worked out why. They simulated incoming invasions with hidden speakers, which played recordings of rival capuchin groups of varying size. Oddly, they found that monkeys are more likely to run away from the speakers when they belonged to larger groups.

Rather than taking strength in the fact that they outnumbered their foes, the monkeys realised that they could afford to cheat. In larger groups, each monkey has a proportionately smaller effect on the outcome of the fight, so each is more likely to desert the battle altogether. If enough do so, the big group can snatch defeat from the jaws of victory. In fact, Crofoot and Gilby found that for every extra group member, the odds that any individual will become a turncoat go up by 25 percent!

As before, location matters: monkeys were almost twice as likely to flee at the edge of their territory as at its centre. When they decide whether to fight or flee, they factor in both where they are and how many friends they have.

The upshot of this is that capuchin territories are remarkably stable. Even though competition is fierce, the monkeys have a natural home-field advantage. When they’re defending the centre of their home ranges, they’re better at converting any numerical advantage into victory. When they’re far from home, and encroaching into another group’s range, even a large force soon loses its advantage as its members flee. This also explains why large groups don’t simply sweep through the jungle, displacing or killing smaller ones on their way.

Now, Crofoot and Gilby want to see if specific groups or other species of primates can overcome the problem of defection to better capitalise on the weight of numbers. The answers could tell us more about our own origins, especially if (as has been suggested) conflict between groups played a central role in our evolution.

Such studies could also inform our behaviour today. The dilemma faced by large groups of monkeys is one that we should all be familiar with. We are facing a multitude of big problems, including a changing climate, a massive loss of other species, and plummeting levels of valuable resources. These problems affect such large groups of people that there are good odds that any one individual will bow out of the fight. If enough do so, defeat will be assured.

Reference: Crofoot & Gilby. 2011. Cheating monkeys undermine group strength in enemy territory. PNAS http://dx.doi.org/10.1073/pnas.1115937109

Image by Kansasphoto

More on cheating:

• Slackers and parasites can sometimes make the best partners
• Cooperating bacteria are vulnerable to slackers
• Green beards, flocs of yeast and the evolution of cooperation
• Punishing slackers and do-gooders

Inbal Ben-Ami Bartal from the University of Chicago found that rats will quickly learn to free a trapped cage-mate, even when they get nothing in return, or when there’s a tasty chocolate distraction around. Bartal thinks that the rats conduct their prison breaks because they empathise with one another. This ability to understand and share the feelings of another individual is found in humans, apes, elephants, dolphins and other intelligent animals. It seems that rats belong in this club too.

This is either a surprise or a retelling of old news, depending on how far back your memory goes. In 1959, the psychologist Russell Church trained a rat to press a lever for food. Then, he connected the lever to the electrified floor of a cage containing another rat. If the first rat pressed the lever, the second one would get a painful shock. That’s not what happened – when the first rat saw what was going on, it forfeited its food and avoided the lever.

Church’s published his results in a provocative paper called “Emotional reactions of rats to the pain of others”, which sparked a flurry of similar studies throughout the 1960s. But the time wasn’t right. Psychologists were mostly interested in what animals did rather than what they felt, and the dominant view of nature red in tooth and claw left little room for cuddly feelings of empathy or altruism. “No one knew what to do with the studies, and they were forgotten,” says Frans de Waal, who studies how animals think.

In later years, the taboo on animal empathy began to lift and people became happier to ascribe it to the wider animal kingdom. In 2006, Dale Langford from McGill University returned to Church’s work and produced more evidence that rats can feel empathy. She showed that mice become more sensitive to pain when they see their cagemates in it.

It seemed that rats are sensitive to each other’s emotions, ‘catching’ them from one another. But Bartal wanted to know if this “emotional contagion” would actually motivate rats to help one another. Would empathy lead to action? Arguably, Church showed as much back in 1959, but psychologists have wondered whether the rats stopped pressing the levers out of concern for their fellows, or out of fear that their own floors would be electrified. Bartal needed a new experiment.

She kept her rats in pairs for two weeks, and then placed one of them in a cage. The trapped rats were clearly stressed – Bartal used a bat detector to show that they were occasionally making high-pitched alarm calls. Their partners could free them by pushing against a restraining door and tipping it over. That’s what they did, although most took a week to learn how.

Bartal found that the rats spent more time exploring the cage, and were more likely to open it, when there was another rat inside. It didn’t matter if the liberated rat got nothing in return. When Bartal changed the set-up so the only exit from the cage led to a different arena, the free rat still opened the door for its colleague, who promptly scurried away.

Even when the rats were faced with a second cage containing delicious chocolate chips, they freed their cage-mate as often as they went for the food. They even shared their chocolate bounty with their liberated pals. “Empathy is a truly powerful motivator, on a par with the desire for chocolates!” says de Waal.

Stephanie Preston, who works on animal emotions, says that Bartal has strengthened the case made by the studies from the 50s and 60s. “As shown previously, the rodents were not only empathically aroused by the emotion of [another rat], they took direct action to help. This is the definition of empathy,” she says.

There are alternative explanations, but none of them are strong. They weren’t just trying to silence the grating alarm calls from their trapped peers, because such calls were too rare to be a potent motivator. They weren’t just curious about the trapped rat, because they still opened the cages if they were very familiar with the animal inside. And they weren’t just looking for something to do for the door mechanism is difficult. The only explanation that really fits the rodents’ actions is that they were trying to end the distress of the trapped rat, or perhaps their own distress at seeing their cage-mate’s plight.

“The study is truly ground-breaking,” adds de Waal. It shows that rodents are not just affected by the emotions of others, but that empathy motivates altruism. Instead of explaining altruism by a cost/benefit calculation, as biologists and economists like to do, we are now entering a distinctly psychological realm of emotions and reactions to the emotions of others. This is where most human altruism finds its motivation and where, as this study suggests, animal altruism does too. In fact, the cost/benefit analysis was carried out long ago by evolution.”

De Waal suggests that the rats’ behaviour is the result of ancient neural circuits that allow mammals to “make the situation of others their own to some degree, thus offering them an emotional stake in it.” These circuits underlie the behaviour of apes, dolphins, elephants, rats, and probably more. De Waal thinks that they originated from the care that mammal mothers offered towards their young, which might explain why female rats (like female chimps and female humans) seem to be more empathic than male ones. In Bartal’s experiment, all the female rats opened doors for a trapped individual, compared to just three-quarters of the males.

Reference: Bartal, Decety & Mason. 2011.  Empathy and Pro-Social Behavior in Rats. 2011. Science http://dx.doi.org/10.1126/science.1210789

Piloting London’s distinctive black cabs (taxis to everyone else) is no easy feat. To earn the privilege, drivers have to pass an intense intellectual ordeal, known charmingly as The Knowledge. Ever since 1865, they’ve had to memorise the location of every street within six miles of Charing Cross – all 25,000 of the capital’s arteries, veins and capillaries. They also need to know the locations of 20,000 landmarks – museums, police stations, theatres, clubs, and more – and 320 routes that connect everything up.

It can take two to four years to learn everything. To prove their skills, prospective drivers make “appearances” at the licencing office, where they have to recite the best route between any two points. The only map they can use is the one in their head. They even have to narrate the details of their journey, complete with passed landmarks, road names, junctions, turns and maybe even traffic lights. Only after successfully doing this, several times over, can they earn a cab driver’s licence.

Given how hard it is, it shouldn’t be surprising that The Knowledge changes the brains of those who acquire it. And for the last 11 years, Eleanor Maguire from University College London has been studying those changes.

In 2000, Maguire showed that one particular part of the brain – the hippocampus – is much larger in London cab drivers than in other people. This seahorse-shaped area lies in the core of the brain, and animal studies had linked it to memory and spatial awareness. Species that store a lot of food tend to have a bigger hippocampus than those without the need to remember any burial sites.

Maguire showed that the same applies to humans. Not only did cab drivers have an unusually large hippocampus, but the size of the area matched the length of their driving careers. Since then, taxi drivers have featured in many of Maguire’s experiments. “They know that they’re special,” she says.”What they’ve achieved when they’re qualified is extremely impressive, so they’re very willing to come and be tested.”

She showed that a driver’s hippocampus is most active when they first plan a route. She found that the hippocampus shrinks back to a normal size once drivers retire. And she found that acquiring The Knowledge comes at a cost – taxi drivers find it more difficult to integrate new routes into their existing maps, and other aspects of their memory seemed to suffer.

An enlarged hippocampus is a rare feature. You don’t see it in doctors who gain vast amounts of knowledge over many years. You don’t see it in memory champions who have trained themselves to remember seemingly impossible lists. You don’t see it in London’s bus drivers who have similar driving skills but work along fixed routes. Among all of these groups, only the London cabbies, with their superb spatial memories, have swollen hippocampi.

These studies strongly suggested that their intensive training was the reason for the changes in the taxi drivers’ brains. They helped to change the decades-old perception of the adult brain as a static organ. Instead, Maguire likens the brain to a muscle – exercise it and it gets stronger. “But of course,” she says, “the real test is to take people before they start training and test them afterwards, to see if there are changes in the hippocampus in the same individual. That would give the best evidence.”

Maguire, and her colleague Katherine Woollett, have done exactly that. They scanned their brains of 79 wannabe drivers who had just started their training. Three to four years later, they did the same thing. By this point, 39 of the trainees – just under half – had earned their licence. The rest had flunked out. The Knowledge is not easily won.

At the start of the study, the trainees had the same memory skills as each other, and 31 men with no aspirations of being cabbies. Everyone’s hippocampus was on a similarly level playing field. The second time round, things had changed. Woollett and Maguire found that the hippocampi of the qualified cabbies had grown in size, especially the back part. They were now significantly larger than those of either the failed trainees or the men who didn’t take part. The cabbies also outperformed their peers on spatial memory tasks.

This is the strongest evidence yet that the training that London cabbies undergo is directly responsible for the changes in their brain. The alternative – that someone with a large hippocampus is more likely to drive a taxi – just doesn’t hold.

Still, there are some unanswered questions. For a start, how exactly does studying for The Knowledge increase the rise of the hippocampus? This small area is one of only two parts of the brain that makes new neurons throughout our adult lives. These extra cells could account for the increased size of a cabbie’s hippocampus. Alternatively, the existing neurons could simply form better connections with one another. Maguire’s next challenge is to tease apart these possibilities.

Another question: why did half of the trainees fail to qualify? Most of them said that they couldn’t afford the time or money, while others cited family obligations. Those could all be valid reasons, but equally, they could be smokescreens that cover a deeper inability. Maguire wonders if genetic differences could give some people a natural edge and others a natural weakness, especially since some genes do affect the size of the hippocampus.

For the moment, Maguire thinks that her work on cab drivers has implications for everyone. “We’re in a situation where people are living longer and often have to retrain or re-educate themselves at various phases in their lives,” she says. “It’s important for people to know that their brains can support that. It’s not the case that your brain structure is fixed.”

She also wonders if her work could one day help people with memory problems, a group that she identifies with. “I’m grossly impaired. I can’t step outside my office without guidance. I keep on having to be talked into places by phone.” She laughs. “It’s very ironic. I’m very motivated to learn how the brain helps you navigate!”

Reference: Woollett & Maguire. 2011. Acquiring ‘‘the Knowledge’’ of London’s Layout Drives Structural Brain Changes. Current Biology http://dx.doi.org/10.1016/j.cub.2011.11.018

For more on the Knowledge and Maguire’s work, see this excellent piece by visiting American, Sally Adee.

Photo by Sammy and the Light

More on the hippocampus:

• New neurons buffer the brains of mice against stress and depressive symptoms
• Exposing the memory engine: the story of PKMzeta
• Drunken monkeys reveal how binge-drinking harms the adolescent brain
• Amnesiacs show that emotions linger long after memories fade
• Child abuse permanently modifies stress genes in brains of suicide victims

My feature about Ehrsson has just come out in Nature. I’m very proud of it, so do check it out. It also comes with a podcast interview with me, and a slideshow of pics that I took on the trip. Here’s the opener:

It is not every day that you are separated from your body and then stabbed in the chest with a kitchen knife.But such experiences are routine in the lab of Henrik Ehrsson, a neuroscientist at the Karolinska Institute in Stockholm, who uses illusions to probe, stretch and displace people’s sense of self. Today, using little more than a video camera, goggles and two sticks, he has convinced me that I am floating a few metres behind my own body. As I see a knife plunging towards my virtual chest, I flinch. Two electrodes on my fingers record the sweat that automatically erupts on my skin, and a nearby laptop plots my spiking fear on a graph.

Out-of-body experiences are just part of Ehrsson’s repertoire. He has convinced people that they have swapped bodies with another person, gained a third arm, shrunk to the size of a doll or grown to giant proportions. The storeroom in his lab is stuffed with mannequins of various sizes, disembodied dolls’ heads, fake hands, cameras, knives and hammers. It looks like a serial killer’s basement. “The other neuroscientists think we’re a little crazy,” Ehrsson admits.

But Ehrsson’s unorthodox apparatus amount to more than cheap trickery. They are part of his quest to understand how people come to experience a sense of self, located within their own bodies. The feeling of body ownership is so ingrained that few people ever think about it — and those scientists and philosophers who do have assumed that it was unassailable.

Finally, many thanks to my editor Helen Pearson, who really helped to knock the piece into shape.

The fire ant is an international pest. It devastates native ants, shorts out electrical equipment, damages crops, and inflicts painful stings. It hails from Argentina, but it was carried to the United States aboard cargo ships that docked at a port in Alabama. That was in the 1930s; since then, this invader has spread throughout the southern states, from California to Florida. The country spends over a billion dollars every year in attempts to stem the invasion.

Now, Shawn Wilder from Texas A&M University has found that their remarkable invasion has been driven by partnerships with local insects. The fire ants run a protection racket for aphids and other bugs, defending them from other attackers. In return, they get honeydew, a sweet nutritious liquid that the bugs excrete, after they suck the juices of plants. They are both farmers and bodyguards.

In Argentina, Wilder found foraging fire ants on around 5 percent of local trees, and they drank honeydew from just 2 percent of the local bug populations. Other species of ants were crowding them out, and dominating the honeydew supplies. By contrast, in America, Wilder found that the fire ants patrol around 40 percent of trees within their range, and control 75 percent of the honeydew-producing bugs. Wilder found the same discrepancy when he placed sugar-water baits throughout the ants’ territories.

The same species that outcompete the fire ants in Argentina can also do so in America, but they’re found in much fewer numbers. The fire ants, facing more relaxed competition, have created a honeydew monopoly.

Wilder confirmed this by measuring the levels of nitrogen isotopes (different versions of the same chemical element) in the ants from both nations. Animals lower down the food chain tend to have lower levels of the heavier nitrogen-15 isotope compared to the lighter nitrogen-14 one, and that’s exactly what Wilder found when he compared the American fire ants to the Argentinean ones. The nitrogen revealed that the American ants are mainly guzzling down on honeydew, while their Argentinean counterparts have to supplement their diet with a lot more hunting.

These extra calories can make all the difference to an incipient fire ant colony. In his lab, when Wilder raised colonies with access to honeydew-making aphids, they grew 20 percent larger than those with no such resources. Out in the field, Wilder also found that fire ants were nearly twice as abundant in areas where they could farm aphids than in areas where he had removed all their potential livestock.

Of course, this isn’t the only secret to the fire ant’s success. It’s also an extremely durable species that can, for example, deal with drought by tunnelling into underground water sources, and cope with floods by forming living rafts (see below). But Wilder says that the sudden unrestricted access to a nutritious, calorie-rich source of food was probably a big factor in its invasive success, at least in the United States. The ants probably encountered a positive cycle: more honeydew meant bigger colonies, which could exercise an even greater stranglehold on honeydew supplies, which meant even bigger colonies.

It’s likely that other invasive insects have also flourished thanks to similar boons. Yellowjacket wasps, and all the most invasive ants, are often found drinking from the other insects. Invaders, it seems, often rely on local collaborators.

Reference: Wilder, Holway, Suarez, LeBrun & Eubanks. 2011. Intercontinental differences in resource use reveal the importance of mutualisms in fire ant invasions. PNAS http://dx.doi.org/10.1073/pnas.1115263108

More on invasive ants:

• Fire ants assemble into living waterproof rafts
• Genetic trick makes black crazy ants adapted for conquest

When we make moral judgments, we do so subtly and selectively. We recognise that explicitly antisocial acts can seem appropriate in the right circumstances. We know that the enemy of our enemy can be our friend. Now, Kiley Hamlin from the University of British Columbia has shown that this capacity for finer social appraisals dates back to infancy – we develop it somewhere between our fifth and eighth months of life.

Hamlin, formerly at Yale University, has a long pedigree in this line of research. Together with Karen Wynn and Paul Bloom, she showed that infants prefer a person who helps others over someone who hinders, even from the tender age of three months. These experiments also showed that infants expect others to behave in the same way – approaching those who help them and avoiding those who harm them. Now, Hamlin has shown that our infant brains can cope with much more nuance than that.

She worked with 64 babies, and showed them a video of a duck hand puppet as it tried to get at a rattle inside a box. This protagonist was aided by a helpful elephant puppet that lifted the lid (first video), but hindered by an antisocial elephant that jumped on the lid and slammed it shut (second video). Next, the babies saw the two elephants playing with a ball and dropping it. Two moose puppets entered the fray – one (the ‘Giver’) would return the ball to the elephant (third video), and the other (the ‘Taker’) would steal it away (fourth video). The babies were then given a choice between the two moose.

Hamlin found that over three-quarters of the five-month-old babies preferred the Giver moose, no matter whether it returned the ball to the helpful elephant or the antisocial one. They were following a simple rule: “helpful moose = good moose”. But the eight-month-old babies were savvier. They largely preferred the Giver moose when it was aiding the helpful elephant, but they chose the Taker when it was took the antisocial elephant’s ball.

In those three months, babies learn to judge an action not simply on whether it helps or harms a person, but also on whether that person deserved it. They prefer characters who help out good puppets, and who punish bad ones. They learn that context matters.

There is, however, another possible explanation. Perhaps the babies were just matching bad for bad. They saw the elephant behaving negatively, so they picked the moose that acted negatively to the elephant. Hamlin disproved this idea in a second experiment. This time, it was the duck that played with the ball and relied on the help of the two moose. Even if the duck had been wronged by an elephant, the babies still preferred the Giver moose.

Finally, Hamlin found that toddlers show the same tendencies themselves. She showed 32 toddlers, aged 19 to 23 months, the same video from before but with dogs standing in for elephants. When she asked the babies to give a treat to one of the dogs, they largely picked the helpful one. When she asked them to take a treat away from a dog, they picked the antisocial one.

Uta Frith, who studies child psychology at UCL, says that Hamlin’s earlier studies were “truly pioneering”. Indeed, many eminent child psychologists, like Jean Piaget, believed that infants only attend to their own needs and thoughts, responding only to an adult’s authority. Hamlin’s 2007study showed the opposite – infants are more than capable of making social judgments. Her new experiments take that conclusion to the next level.

“The experiments make clear that young children do not merely put positive and negative values on agents on the basis of their experience, and prefer the goodie,” says Frith. “Instead, they can tell the difference between appropriate reward and punishment according to the context. To me this says that toddlers already have more or less adult moral understanding. Isn’t this amazing? I don’t know in what way adults would react in the same situation in a more sophisticated way.”

Reference: Hamlin, Wynn, Bloom & Mahajan. 2011. How infants and toddlers react to antisocial others. PNAS http://dx.doi.org/10.1073/pnas.1110306108

More on child development:

• Children share when they work together, chimps do not
• The development of fairness – egalitarian children grow into meritocratic teens
• Infants match human words to human faces and monkey calls to monkey faces (but not quacks to duck faces)
• Native language shapes the melody of a newborn baby’s cry
• Five-month-old babies prefer their own languages and shun foreign accents
• Self-control in childhood predicts health and wealth in adulthood
• Children learn to share by age 7-8