2020-04-02_Science_Illustrated

highly surprising. Overall, they show that the IQ of the

eight-year-olds matches the IQ of the adopted mother

to some extent. At this age, the kids’ IQ is determined

both by nature and nurture. But this changes as they

grow older. At 18, their IQ is much closer to their biolog-

ical mother’s, and equivalently different from their

adoptive mother’s. The results made John Loehlin

conclude: “Shared family environment has an appreci-

able effect when children are small, but this becomes

minor by the time they are late adolescents.”

This implies that our intelligence is determined by a

mixture of nature and nurture, but the effects of nurture

level out as we grow older. If you score high in an IQ test

as kids, thank your parents for a good upbringing. If you

score high as an adult, thank them for their genes.

It also means that there is no formula that can be

applied to say to what extent differences of intelligence

are due to nature or nurture. And of course scientists

disagree a lot, some believing that nature is responsible

for 30% of differences, while others claim that the

percentage is as high as 80%.

A generalisation too far

Interest in intelligence research exploded in the US

in the mid-1990s, following the publication of a

controversial book called ‘The Bell Curve’, writ-

ten by psychologist Richard Herrnstein and

sociologist Charles Murray. The title refers to

the bell-shaped curve which indicates the

normal distribution of intelligence in the

population. Through almost 900 pages, the

authors delved into statistics concerning

links between IQ and different social

conditions. The book included summaries

claiming that low intelligence is a domi-

nant feature in groups of people character-

ised by poverty, high unemployment, early

divorces, a low level of education, and a high

crime rate. The authors went one step further,

commenting on the links and problems, and suggest-

ing possible political solutions. They believed that the

government should take steps to limit the number of

children with low intelligence in any given group.

The book’s content about the average intelligence of

different ethnic groups – blacks, white, and people of

Latin American descent – was even more controversial.

The statistics implied a difference, with blacks scoring

averagely low in IQ tests, and whites scoring the highest.

To many who read “The Bell Curve”, Herrnstein and

Murray’s belief that the difference was due to both

nature and nurture seemed to be an effort to revive an

ideology by the name of eugenics. The debate about

the book’s contents became as emotional as it was

academic, some attacking the book for being racist,

others for its statistical methods, and still others for

its interpretations of the research results. When it

comes to the IQ of different ethnic groups, most

scientists acknowledge a statistical difference, but

emphasise that it can be explained by environmen-

tal and social factors, not necessarily by the form of

genetic inheritance. Moreover, they point out that the

difference of IQ within each group is much larger than

According to

some scientists,

intelligence can

be divided into

two types:

liquid and

crystallised.

The former

is based on the

ability to solve

new problems,

whereas the

latter relies on

experience. The

two change over a

lifetime, while the

general IQ remains

constant from

the age of 20.

Over a period of 100 years, scientiests have

unsuccessfully measured, weighed, and scanned

the human brain to find physical parameters that

can be linked with intelligence. Exterior indicators

such as brain shape, size, and weight apparently

mean very little to our IQ.

Intelligence hides

from brain researchers

The number of cells does not matter

Our intellectual capacity is spread across all of

the cerebral cortex, but it is apparently not very

important how many brain cells we have. Women

are just as intelligent as men, although on

average they have four billion fewer brain cells.

Size means almost nothing

Some brain researchers have found a link

between IQ and brain size, but it is very

modest and doesn’t impact on comparisons of

intelligence between men and women.

Weight is not important at all

On average, men have 130g more brain than

women, but that does not give them an upper

hand when it comes to IQ. The weight also

seems unimportant to individuals of the same

sex. Einstein’s brain weighed only 1230g, which

is less than the female average.

Women:

1,245 g

Men:

1,375 g

020

Crystallised

intelligence

Liquid intelligence

General IQ

40 60

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