How was the heritability of IQ tested?

Question

I have read that IQ is highly heritable.

How could this be tested?

For honest testing, it seems to me that only children who were abandoned should have been tested, to exclude education effect. Was it really done?

UPDATE

Intelligence is one of the traits, that can be inherited both by genes and by education. You can't educate your children to have blue eyes, but you can educate them to count or to read.

So, to study heritability of intelligence, educational inheritance should be excluded.

I know that usual way to study heritability is to compare variability for monozygotic and dizigotic twins. Being sufficient for other traits, this method is highly insufficient for intelligence (which is obvious). Having identical perception system, monozygotic twins will respond identically to an education, so they will be more closely educated, even if IQ is completely independent of genes.

So, to study IQ heritability, subjects should be studied under different education conditions. I.e. they should be abandoned by parents and/or adopted separately.

My question is: was this done or not?

If not, then the question is, how education effects were eliminated?

Also I would like to know, if my reasoning about why just studying twins is insufficient is wrong.

Answer 1

I don't really know what you mean by "abandoned children". Adopted children? Children in the wild? Have you read the Wikipedia article on the subject? If you want to know how it was studied, just take a look at the references, it's pretty straightforward.

Heritability simply means how much variation in a trait is due to genetics, which is usually easiest to test in monozygotic twins. Here's a good example where they did just that - they compared identical (monozygotic) and fraternal (dizygotic) twins raised apart or separately, finding:

General cognitive ability yielded a heritability estimate of about .80 in two assessments 3 years apart as part of the Swedish Adoption/Twin Study of Aging. This is one of the highest heritabilities reported for a behavioral trait. Across the two ages, average heritabilities are about .60 for verbal tests, .50 for spatial and speed-of-processing tests, and .40 for memory tests. For general cognitive ability, the phenotypic stability across the 3 years is .92 and stable genetic factors account for nearly 90% this stability.

Here's another example doing basically the same thing. The rest of the enWiki article gives a good breakdown of all the various factors involved in making this such a complex issue, as well as what was studied.

Answer 2

A classic study of IQ heritability is Skodak & Skeels (1949). They studied heritability of IQ in children who were adopted at a very young age, with IQ data of the children, of their biological mothers, and of their adoptive mothers. It is important to keep in mind that heritability is a measure of the proportion of variance that is attributable to genetic inheritance in the population; it is formally the ratio between genetic variance in a population to the phenotypic variance in a population. The above mentioned study found a high heritability between biological mothers and their children (0.88), but no heritability between adoptive mothers and their children. Yet the mean IQ of children were similar to adoptive mothers, and much higher than their biological mothers.

This is not a contradiction. High heritability does not mean that environmental factors are not important. Indeed, environmental factors (such as upbringing, education, socioeconomic status) may have a large effect on mean values, hence why the children had IQs similar to their adoptive mothers. However, heritability explains variance not mean; in the above study, there is a high correlation between where children fell on the normal distribution of IQ scores among the children and where their biological mothers fell on the distribution of biological mothers, but no correlation with where their adoptive mothers fell on the distribution of adoptive mothers.