Another method for studying the role of genetics is adoption studies. The Colorado Adoption Project is an ongoing longitudinal study of children separated from their biological parents in their first month of life; Plomin et al. found that, as these children get older, their intelligence becomes more and more like that of their biological parents and less like that of their adoptive parents. This indicates that biological factors are significant, but have a latent effect. This finding is duplicated by the Texas Adoption Project, which found no correlation between adopted children and their unrelated siblings by the age of 18. Thus, it may be that environmental factors are only significant in childhood, and through puberty it is the genetic factors that shape the person's intelligence.
If intelligence is the result of genetics, it follows that there must be some gene or collection of genes that causes a higher IQ. A study of children with 'ultra-high' IQ scores revealed that a particular variant of a specific gene on chromosome 6 was twice as common in these children than in those with average IQs. However, even in the high IQ sample, the rate was only 46%, so there must have been either other genes or other factors involved in their intelligence.
The implications of the role of genes in intelligence raise important ethical issues. Jensen (1969) suggested that genes account for 80% of our intelligence, and furthermore stated that the mean IQ score for white Americans was 15 points higher than for black Americans, and that this was unchangeable. This would indicate that entire races of people are intellectually inferior to others, which provides support for the eugenics movement, therefore obviously being ethically objectionable.
Such assumptions about race being such a large factor in intelligence may result from a lack of reliability of IQ tests, and indeed the concept of an 'intelligence quotient'. IQ tests are frequently criticised of containing several types of bias, and particularly gender and culture bias. In some cultures, forms of intelligence that require fast, logical thought are not considered as important as slow, considered conclusions to arguments, for example. Additionally, many IQ tests require literacy or numeracy, which may not necessarily reflect cognitive ability. This is significant with regard to studies like that of Jensen, above, because different races are often separated by geographical, cultural and sub-cultural barriers. Black Americans are, statistically, much lower in socio-economic status than white Americans, so it may be this environmental factor that causes their IQ to appear lower on tests.
All of the above research, however, assumes that genetics directly affect intelligence, but this may not be so: they may affect it indirectly. For example, if a person is genetically predisposed to have a personality such that they respond positively to others, it may be that others respond positively to their questioning about the world, and therefore their intelligence becomes more developed. This 'microenvironment' may result in a secure attachment with a parent, which itself may result in a higher IQ. As it also relies on others, it would also indicate that environmental factors are significant, as it is a form of reciprocal determinism, which may account for correlations between monozygotic twins of less than 100%. This is supported by Broungart et al., who found that the correlation between the IQ of children and their parents was higher when the parents were biologically related than otherwise, indicating a definite, but indirect, genetic link.