Welcome to Your Brain (24 page)

They find that individuality appears to be a biological imperative that may be essential to survival

strategies for any species. The research has illuminated what personality is good for and how it can

be shaped by inheritance, development, and experience. It has even produced some early glimmerings

of how brain mechanisms generate animal—and human—personality.

Many traditional psychologists have avoided the study of differences among animals altogether. A

pioneer in animal behavior research, B. F. Skinner made a point of giving tests to animals under

conditions that made the responses as reliable as possible. He designed his famous Skinner box to

remove any distracting stimuli that might lead to environmental variation. Skinner’s idea of a perfect

experiment was one in which there was no variation from individual to individual; in principle, if you

had a good experiment with one animal, you might use a second one only to make sure everything was

okay.

There is a plausible reason for discounting talk of personality, not only in animals, but even in

people. We constantly map our own actions to our individual motives and preferences, and we tend to

assign similar motives and preferences to the actions of others. But this is a slippery slope. As

pointed out in
Chapter 1,
your brain is constantly lying to you about your own reasons for acting. We

inadvertently create mental models of how things work expressed in terms of agency, even when the

things in question are inanimate objects. For instance, it is common to describe a car as

temperamental, or a house as personal and inviting. Yet nobody would attribute literal personality to

these objects.

Ethologists wrestle with this problem continually. Their answer is to work with behaviors that

are directly observable. Did the animal attack? Did it retreat? Did it curl up in a corner? In some

sense, Skinner was just as focused on observing behaviors that were quantifiable. But ethologists’

interest in differences has enabled them to catalog individual traits and try to understand the reasons

for them.

One very striking finding is that not only do animals have individual personalities, but individuals

can be categorized according to the same groupings and qualities that we use in classifying human

personality. In one pioneering study at the Seattle Aquarium, researchers were able to break down

octopus temperament into three principal dimensions: activity, reactivity, and avoidance. These

measures did a good job of describing how the animals would react in various situations that

included a human observer sticking her face into the tank, waving a test tube cleaning brush near the

animal, or dropping a tasty crab into the water. Over time, and in controlled situations, researchers

were able to reliably predict the tendency of individual octopuses to attack, retreat, or remain calm.

The variability in temperament in octopuses and many other species raises the question of what

evolutionary sense it makes for a species as a whole if the natural behavioral tendencies—the

temperament—of individuals vary. One possibility is that different personalities can adapt to

different niches in the environment. For instance, being daring might get an animal to the front of the

line in grabbing the food, but if many dangerous predators are nearby, that animal would also be at

greater risk of being eaten. In such a situation, the stay-at-homes could lie low, then come out to cadge

a few scraps—and live to see another day. Likewise, extroverted people may get more dates, but they

also have more accidents and therefore end up in the hospital more often. Finally, consider an

extreme example, the female North American fishing spider. Some fishing spiders are extremely

aggressive hunters and are always first to grab passing food. But these same females have trouble

during the mating season, when they can’t keep their legs off of their suitors—and then eat the poor

guys before they have the chance to mate. Oops.

Variation may be a strategy that helps a species to survive in an ever-changing world. The world

changes much faster than species do as a whole, as adaptation through genetic change takes many

generations. Sexual reproduction rescues us from this plight. Each individual of a species takes DNA

from his or her parents and combines it to make a new, genetically unique combination. The resulting

variation can help to ensure that somebody makes it to the next generation.

As in humans, the individual signatures of octopus behavior are not fixed in time. Octopus

temperament is malleable from the age of three weeks to six weeks, when temperament (as measured

by the three-dimensional scale) varies a lot. Over this period, aggressive animals became shy, and

excitable animals became phlegmatic. In humans, personality is most changeable before the age of

thirty, after which we tend to settle into a pattern that lasts for many years.

The effects of genetics and environment on personality can be separated by examining animal

populations. For example, in dairy goat siblings that were split up into two groups, one reared by

humans and the other reared by mother goats, the relative rank order of timidity was the same within

each group: the most timid goats in each group tended to be siblings, with a general tendency for the

human-reared goats to be less timid. This finding indicates that temperamental traits start from an

inborn tendency but can also be influenced by environmental factors such as upbringing.

Did you know? Domesticating the brain

The earliest known domestication of animals dates to more than ten thousand years ago,

when dog and human remains first appeared in the same burial sites. It is not known

whether domestication initially arose from the gradual selection of more pliant offspring—

for instance, by feeding the wolves who were least fearful of a fire—or by selective

breeding of captive animals. One twentieth-century experiment by Russian geneticist

Dmitry Belyaev suggests that purposeful breeding can lead to especially fast changes in

behavior. In his experiment, foxes were selected for docility and only the friendliest pups

selected for later breeding. The result of more than thirty generations of such selection was

a colony of foxes so friendly that pups actively competed for human attention.

A number of physical attributes often accompany domestication. As Charles Darwin

noted long ago, domesticated animals tend to have more floppy ears, wavy or curly hair,

and shorter tails than their wild cousins. The recurrent appearance of these traits in

different species suggests that breeding for tameness selects for a whole constellation of

related traits at the same time. One notable consequence of domestication is relative

shrinkage in brain size. In domesticated pigs and chickens, forebrain structures occupy

about one-tenth less of the brain than they do in the wild. A mechanism that could account

for many of these changes is a tendency for juvenile traits to be retained in adults. In other

words, domestic breeding may select for slowed development.

Many studies of personality and the brain have focused on dopamine and serotonin, two

neurotransmitters secreted by cells in the midbrain that are important in regulating the activity of the

nervous system. These neurotransmitters are released by nerve terminals throughout the brain and are

cleared out by dopamine and serotonin transporters, molecular pumps that suck them back into cells,

to be used again or broken down.

How these transmitters are handled can influence the personality of humans and nonhuman

animals. For example, in people, studies of identical twins show that about half of the variation in

anxiety-related personality traits is inherited. Some of this variation may be due to differences in the

action of serotonin. Mice that have been genetically modified to lack a particular type of serotonin

receptor show far less anxiety in conflict situations than their normal cousins. In humans, genetic

evidence suggests that anxiety-related personality traits may be associated with a shortage of a

specific protein that is responsible for the reuptake of serotonin. The influence of the serotonin

reuptake protein accounts for somewhat less than a tenth of the inheritable variation in anxiety. In both

humans and mice, the relationship between serotonin reuptake and mood can be manipulated. Prozac

treats anxiety and depression by inhibiting this serotonin reuptake protein.

Another example of a personality trait that has attracted much research is the tendency to seek out

novel experiences. Unsurprisingly, this trait varies inversely with the tendency to avoid harm. Both

novelty seeking and harm avoidance are associated with particular types of dopamine receptors, not

only in people but also in thoroughbred horses.

Genetic traits such as dopamine and serotonin activity predict personality only to a small degree.

The findings are still interesting, though, because they point toward the possibility that we may one

day understand how personality traits are determined, both by genes and by environment. Also, even

if personality and mood are a black box with a dozen knobs on it, our ability to identify and turn one

of them may be behind the effects of drugs such as Prozac.

At the same time, the weakness of these associations does raise the question of how personality

can be so visibly inheritable, yet individual genes for personality are so hard to identify. The

cumulative picture that emerges from studies of the genetics of personality is that inborn aspects of

personality traits are polygenic, meaning that they are constructed from the action of many genes,

perhaps hundreds. For this reason, even in the best cases, genetic traits such as particular receptor

types have so far only been able to account for a small fraction of the variation seen from individual

to individual.

From an evolutionary standpoint, the polygenic nature of personality may be a good thing. Sexual

reproduction mixes up the genetic contribution from two parents in unpredictable ways. This allows

the dice to be rolled again and again, yielding a range of personalities distributed over the whole

spectrum, generation after generation.

The omnipresence of variation in temperament in animals and people leads us to ask whether

what we consider abnormal may vary according to the times and local culture. Someone who is

considered hopelessly obsessive-compulsive in Papua New Guinea might be a harmless collector of

clocks in Switzerland. Even extreme individuals may help our species survive in times of great need.

The best warriors of Genghis Khan’s army might be locked up as bloodthirsty psychopaths today. So

the next time you encounter another diagnosis of attention deficit disorder, just think what a dandy

hunter-gatherer that person would have made.

Chapter 20

Sex, Love, and Pair-Bonding

When we talk about mating in nonhuman animals, we’re supposed to call it “pair-bonding” rather

than love. But if you watched a mated pair of prairie voles together, their behavior would look a lot

like love to you. The prairie vole, a small brown burrowing rodent, stays with the same mate for life

(which is unusual, since only 3 to 5 percent of mammals are monogamous). Both parents care for the

offspring, and prairie voles that lose their mates typically refuse to take another partner.

In contrast, the meadow vole is solitary and has promiscuous breeding habits. By comparing the