2020-04-02_Science_Illustrated

confirmed the same trend, causing him to conclude that

“Whether you live to collect your old-age pension

depends in part on your IQ at age 11.”

Though there are several factors, some reasons why

our intelligence quotient as kids might predict our life-

span are not hard to guess. As Deary suggests, part of the

explanation is that higher intelligence in kids often

leads to better academic education later in life, which in

turn will predicate healthier and safer environments for

the rest of their lives.

The slow curve of age

Although our IQ from a very early age seems to affect us

throughout life, this does not mean it is a constant.

Some basic mental capabilities can improve; others can

become impaired. Again individuals will vary but studies

indicate a general trend. When it comes to solving

language problems, we generally improve from age 25 to

around 40. Subsequently, we remain at the same level up

until the age of 80. However with

logic problems, such as conclud-

ing a rule based on a few exam-

ples, it goes the other way, our

performance generally deterio-

rating from age 25 to 80.

These two types of capacities

are examples of what psycholo-

gists have named “crystallised

intelligence” and “liquid intelli-

gence”. When we use our crys-

tallised intelligence, we benefit

from knowledge and experience

that are solidly anchored in the

brain. When we use our liquid

intelligence, we do not use

previous knowledge, but must

reason our way forward from a

new and unknown situation.

Our general IQ can also

change over time, although

studies show major individual

differences, so that in some

people it improves, and in others

it deteriorates. But the general trend is that our g factor

becomes weaker with age. It remains a mystery why

that is so, but scientists suspect that there is a physical

explanation: our brain cells simply function more slowly

with age.

Several experiments, including those carried out by

Ian Deary in 2017, show that there is a link between

response time and measured intelligence. People with a

high IQ are averagely quicker. And the connection seems

clearer the older the test subjects are.

Speed of response might therefore be a clear cause

for the g factor generally deteriorating with age. But

speed may also be a small part of the explanation for

why some people have a higher IQ than others in the

first place. The speed at which our brain cells communi-

cate with each other depends on the brain’s ‘white

matter’, bundles of nerves that link different areas of the

cerebral cortex. The nerves are insulated by myelin

sheaths, and the more sturdy these are, the faster the

signals travel through the nerves. With age, the myelin

sheaths become damaged, which might explain why the

nerve cells communicate more slowly. Likewise, extra

sturdy myelin sheaths could benefit some of us IQ-wise.

Scientists emphasise that our individual intelligence

is very complex, and that the speed of the nerve cells

probably explains only some of the differences between

us. But the theory is at least a concrete link between

intelligence and the physical brain.

Other efforts to find physical explanations of IQ

differences have not produced many answers. The size

of the brain does not seem to mean very much. Since the

1990s, when it became possible to measure the volume

of living people’s brains by means of MRI scans, several

comparisons have been made between brain size and

IQ. Some of them demonstrated that a big brain means

a slightly higher likelihood of the owner having a higher

IQ, but the link is not very clear. The same is true for the

brain’s weight, and for the number of brain cells.

While brain researchers have been seeking largely in

vain for intelligence in the physical brain, psychologists

have used other methods in recent decades to find out

why some of us are more intelligent than others. And

one major area of study is the age-old question of nature

versus nurture.

Separated at birth

When Gerald Levey met Mark Newman, it was

like seeing himself. The bald pate, whiskers, the

moustache – even the glasses were the same.

The two middle-aged men had never met

before, but they had an incredible amount in

common. They both worked in lawn mainten-

ance; they were both voluntary firefighters. They

drank the same brand of beer, carried their

keys in the same way, and liked the same

western films.

Gerald and Mark are identical twins, but

were adopted by two different families after

they were born, and so they had been selected by

psychologists from the Minnesota Center for Twin and

Family Research in the US. Since 1979, scientists from

the centre have summoned more than 100 pairs of twins

like Gerald and Mark to study the link between intelli-

gence and nature and nurture. Identical twins have the

same genes, so they are perfect test subjects for scien-

tists – particularly if they grew up apart.

Over time scientists have made so many twins take

IQ tests that they can identify the general trends of the

influence of nature and nurture on intelligence. The

results are remarkable. Twins who live their entire lives

apart score almost the same in IQ tests, the difference

so slight that it is close to the difference between twins

who grew up together. These results indicate that

nature, i.e. the genes, means a great deal to intelligence.

Other studies carried out by the Texas Adoption

Project in the US have investigated the effect of genes in

a different way. Scientists headed by Professor John

Loehlin IQ-tested kids who were adopted at birth, while

at the same time testing their biological mothers and

adoptive mothers. The tests were made twice – when

the kids were 8 and 18 years old. Again the results are

Shared family

environment has an

appreciable effect on

children’s IQ, but

becomes minor by

the time they are

late adolescents.

INTELLIGENCE RESEARCHER JOHN LOEHLIN

on nurture’s limited influence on our IQ.

72 | SCIENCE ILLUSTRATED

HUMANS INTELLIGENCE