Welcome to Your Brain (19 page)

by New Zealand political scientist James R. Flynn. Using data from twenty countries from around the

world, Flynn examined performance on standardized IQ tests over time. He found that, within each

country, the average scores were steadily higher for people who were born in later years—increasing

about three IQ points per decade. In some nations, such as Denmark and Israel, IQ scores rose even

faster, about twenty points over thirty years—little more than a single generation. For instance, in

verbal and performance IQ, an average Danish twelve-year-old in 1982 beat the average scores of a

fourteen-year-old from his parents’ generation in 1952.

Did you know? Understanding nature versus nurture

What determines intelligence—genetics or your environment? The simple answer is

both, but let’s examine it a little. Genes have no effect without an environment, and vice

versa. Both must interact during a child’s development. The more interesting question is

how they interact.

For many characteristics, your genes basically set an upper limit on your development.

Take height, for instance. If we imagine two children with the same genes (like identical

twins), the one who isn’t fed enough protein while growing up (let’s call him Tom) will

end up shorter than the one who gets good nutrition (Mike). On the other hand, once Mike’s

basic nutritional needs have been met, stuffing him full of extra fish and chicken won’t make

him grow any taller, because he’s hit his genetic limit. Instead, he’ll just get fat. A third kid,

Jeff, whose parents have passed along height genes with more potential but don’t feed him

as well, may end up the same height as Mike. Immigrants who move from poorer countries

to richer ones often see their children grow much taller than they are. By the same process,

economic development can increase the average height of a population.

Sam has seen this effect in his own family. He is six foot one, several inches taller than

anyone in his parents’ generation, all of whom grew up in prerevolutionary China. His

brother Ed, at six foot six, towers over them all; his height is unheard of in the previous

generation. As native-born Americans, they are examples of the height benefits that come

from living in a highly developed country.

Intelligence works in a similar way, except that the environmental influences on its

development are more complicated and less understood. Basic nutrition is important for

any kind of growth, but brain development is probably also influenced by other factors, like

social experience and intellectual stimulation. But by the same token, once the environment

meets a certain standard of quality—albeit one that’s not well defined—no amount of extra

nutrients or stimulation will increase a child’s intelligence beyond the natural limit

imposed by genetics.

Changes in IQ over time imply that intelligence tests don’t simply measure some pure, inborn

capacity, but also track the effects of the environmental surroundings in which a person matures.

Better nutrition and health can lead to better brain growth, and a more stimulating environment may

also enhance brain development and function. Indeed, since we are highly social animals, these

factors may be intensified by social interaction with other individuals who are also developmentally

accelerated, leading to a positive feedback effect—and even more improved performance. Because

of better nutrition and a more stimulating environment, it is entirely possible that people’s brains

today are, on average, more sophisticated than they were a hundred years ago.

Some evidence suggests that this effect has begun to level off. In Denmark, the nation with the

largest past gains, IQ scores have stopped increasing in recent years. One possibility is that

environmental effects can limit brain development, but only when resources are scarce (see
Did you

know? Understanding nature versus nurture
). In other words, as the number of people who are poor

or resource deprived decreases, the average IQ increases. This idea is supported by a recent study of

Spanish children, which examined intelligence gains in the population over a thirty-year period. The

IQ scores among the lowest-scoring children went up the most, with almost no gain in the top half of

the population. Further support for this idea can be found in studies in the U.S., which show that, at

poorer levels of society, educational achievement is correlated with the resources available in

schools, but at richer levels, educational achievement is more strongly correlated with heredity and

home environment.

However, all this progress does not mean that our brains are evolving. Instead, because the Flynn

effect has occurred steadily over just a few decades, it cannot possibly be true evolution. Evolution

usually refers to changes in genes that are passed on to offspring and would therefore require at least

one round of reproduction and selection. This would lead to hereditary changes, so that a person born

with the advantageous genes would eventually outperform other people brought up in the same

environment.

An important thing to understand is that natural selection works through practical outcomes. It

doesn’t matter whether an animal knows how to find food because it’s got an automatic program for

food-finding tattooed on its brain at birth, or whether it is good at learning from its early experiences

to get better at foraging. Either way, if that animal gets enough to eat, it will survive and be more

likely to reproduce. For this reason, natural selection has produced brains that enable their owners to

survive in the environment around them. Different animals may succeed by being adept at social

interactions, or by being good at learning to survive in different environments. So nature versus

nurture is the wrong debate; selection promotes genes that are especially good at getting along with

their environments.

Did you know? Machiavellian intelligence—a brain arms race?

Primates are social—and mean. It’s true for monkeys, and it’s true for apes, including

humans. We live in groups, compete with one another for food and mates, and are

constantly forming and breaking alliances. The reasoning behind these social relationships

can get quite convoluted, starting with “I like you; you like me” and ending up with “You

pretend to like me when we are in front of her” and even “You and she might take my

banana when I am not looking.” It’s a jungle out there.

Some have suggested that constant social competition is a main factor driving brain

evolution in primates. Within the history of a species, social maneuvering over many

generations may favor the selection of individuals with more mental firepower. This would

lead to a brain arms race, in which increases in some animals’ brain size would create

pressure on other members in the species to keep up. Indeed, our species devotes more of

its brain mass to cerebral cortex than any other species, about 76 percent. Chimpanzees are

in second place at 72 percent, gorillas in third at 68 percent. Dolphins, although they have

large brains in absolute terms, are considerably behind at 60 percent. In our case, the extra

cortical volume turns out to be good for many things, like language and making tools.

Increased brain size could also open up new niches in the environment where a species

may thrive. For example, though chimpanzees and gorillas are restricted to certain parts of

Africa, humans were able to find a way through the geographic bottleneck leading from

Africa to other parts of the world—and then adapt to a wide variety of conditions.

When people ask if the brain is still evolving, they often mean to ask whether the genetic

mechanisms that determine brain size or structure are changing. This is harder to answer because it

can be many generations before any change at the evolutionary level becomes visible.

Human evolution by natural selection is hard to observe within a person’s lifetime, but it is

possible to study in animals with a short life cycle, so that many generations fit into a single lifetime

of a human observer. For instance, in the Galápagos Islands, where food supply and weather vary

strongly from season to season, finches with different beak types survive depending on the type and

location of food available. Finches grow to adulthood and reproduce in just a few years. Over

multiple generations, the range of beak types can change, moving toward long and narrow or short and

stubby, depending on what is better for obtaining food. These changes have been seen in times as

short as a single decade.

For natural selection to occur, individuals with a certain characteristic must have more offspring

than individuals lacking that characteristic. Selection for differences in brain function is likely to be

gradual; it may take millenia before any changes in intelligence become evident. Promoting the Flynn

effect, which works much faster, is a better bet for improving our species—or at least a bet with a

more immediate payoff.

If evolutionary change eventually does occur, however, it will be a continuation of processes

already at work in the history of our species. There is evidence for relatively recent evolution of

some of the genes that drive brain development (“recent” in evolutionary terms, meaning over the last

ten thousand years). Two genes involved in brain development,
Microcephalin
and
ASPM
, have been

studied in individuals around the world. These genes were originally discovered because they lead to

severe defects in brain size or structure when missing or damaged. Persons with defective

Microcephalin
or
ASPM
are physically normal except that their brains are tiny; as a result, they are

severely mentally retarded. This defect suggests that the proteins encoded by
Microcephalin
and

ASPM
are necessary in some way for normal development. This led to the speculation that the

functionality of these proteins could also vary within the general population, and therefore lead to

variation in brain size among individuals.

A team of researchers working with DNA from over a thousand people around the globe found

that particular versions of
Microcephalin
and
ASPM
are inherited much more frequently than would

be expected. This suggests that natural selection is at work. Based on comparisons with the rate of

change in the rest of the genome over time, newer versions of the genes first appeared in the human

population between six thousand and thirty-seven thousand years ago. The time is not known more

precisely because DNA from that long ago has not been tested. Since generation times are typically

fifteen to twenty years, these changes represent the cumulative outcome of hundreds to thousands of

generations of selection.

It is also not known what the preferred versions of these genes are doing for people. So far, no

correspondence has been found between gene version and brain size among normal humans,

suggesting that brain size is determined by many additional factors. It is possible that these genes give

some other advantage, such as a lower chance of developing a brain defect. The genes could even be

involved in the development of other organs. Like the Flynn effect, defects in these genes may be a

form of deprivation. In any case, the mechanisms that have driven increases in normal brain size are

yet to be determined. Whatever these genes are doing, they fit into a larger story in which

evolutionary genetic change in brain development takes thousands of years to accumulate. So don’t

hold your breath!

Part Four

Your Emotional Brain

The Weather in Your Brain: Emotions

Did I Pack Everything? Anxiety

Happiness and How We Find It

What’s It Like in There? Personality

Sex, Love, and Pair-Bonding

Chapter 16

The Weather in Your Brain: Emotions

Most people assume that emotions interfere with our ability to make sensible choices—but that’s

not right. Emotions (unlike moods) occur in response to events in the world and keep our brains

focused on critical information, from the threat of physical harm to social opportunities. Emotions

motivate us to shape our behavior to gain what we desire and avoid what we fear.