MadSci Network: Genetics
Query:

Re: What is the heritability of intellegence in humans?

Date: Sat May 23 18:55:48 1998
Posted By: Amy Caudy, Undergraduate, Biology, Washington University
Area of science: Genetics
ID: 893302307.Ge
Message:

Well, this question is the subject of hot debate. While the involvement of genetic factors in intelligence is pretty much unquestioned at this point, the precise involvement is very difficult to pin down.

Most studies of the subject have dealt with identical (monozygotic) twins. These people have identical genomes, so if intelligence was completely determined by genes, the twins should have identical intelligence. However, this isn't the case, because the environment plays a crucial role.

It is also crucial to remember the definition of heritability. Heritability is the proportion of variation of a trait which can be attributed to genetic factors as opposed to environmental ones. That is, if you are 10 IQ points away from the mean, and heritability is .8 (80%) then 8 points were due to intelligence. Heritability doesn't mean that 80 points of an IQ of 100 are due to genetics, it merely describes the distance away from the mean.

Here's a way to think of heritability: If you were to take identical twins and put one in a great environment, then they will be a certain amount above or below the mean for kids in that enviroment. The other twin could be in a poor environment, and their IQ would be lower. However, they'd be the same amount above or below the mean as their twin. They'd just be at a different mean.

Heritability measurements are dependent on another two factors: the genetic variation and environmental similarities within a population. Finally, the measurement of heritability in humans is difficult because families tend to be fairly small.

Given all this, many different scientists have tried to assign heritability to intelligence. About 3 times as many papers are written arguing about the results as there are papers with results, so I'll give you a few results and some references for more information.

      AUTHOR: Lynn, Richard; Hattori, Ken
       TITLE: The heritability of intelligence in Japan.
      SOURCE: Behavior Genetics
        DATE: 1990 Jul Vol 20(4) 545-546

They found a heritability of intelligence (the abstract doesn't mention the measure they used) of .582 in a study of children under 12.


      AUTHOR: Swan, Gary E.; Carmelli, Dorit; Reed, Terry; Harshfield,
              Gregory A. et al
       TITLE: Heritability of cognitive performance in aging twins: The
              National Heart, Lung, and Blood Institute twin study.
      SOURCE: Archives of Neurology
        DATE: 1990 Mar Vol 47(3) 259-262

The found a heritability of .68 in a study of aging twins (averate 63 years old), using the Digit Symbol (DS) subtest of the Wechsler Adult Intelligence Scale--Revised.


      AUTHOR: Devlin, B.; Daniels, Michael; Roeder, Kathryn
CORP. SOURCE: U Pittsburgh, School of Medicine, Dept of Psychiatry,
              Pittsburgh, PA, US
       TITLE: The heritability of IQ.
      SOURCE: Nature
        DATE: 1997 Jul Vol 388(6641) 468-471

These people found heritability of less than .5 (50%) for intelligence and propose a new model, where the maternal environment (i.e. whether sibilings share the womb or are born separately) is more important than genes.


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