twin and adoption studies very clearly
Twin, adoption, and twins-reared-apart studies may seem straightforward and on solid ground, but closer scrutiny reveals problems with one thing in common -- underestimate role of environmental factors and exaggerate role of genes.
Twin studies are based on assumption that while identical and non-id twins share different proportions of genes, each pair of twins will share the same environment. It is known as the equal environment assumption.
However, experiences of identical and non-identical twins, in terms of shared environment, are very different. Identicals tend to experience more similar environments for no other reason than that they are seen as identical by others. They are treated as more similar to eachother by people around them, more likely to be dressed the same and share the same resources and opportunities etc.
This is important because some of the observed differences between identical and non-identical twins, in terms of IQ similarity, may not be due to a greater proportion of shared genes, they may be due to the fact that identical twins tend to undergo a much more similar experience environment-wise than non-identical twins.
Similar methodological problems found in twins-reared-apart studies. Assumption is that twins reared apart will be raised in different environments. They are assumed to share all the genes but none of the environment. Yet, a lot of the twins used in these studies shared quite a lot of their environment -- first they spent some of their childhood together. Twins rarely adopted at birth so have been together a number of years before separation, sometimes for as long as 11 years, so shared environment and experiences during key psychological development periods.
Also, some of the twins were not adopted through welfare orgs/social services but were simply raised in different branches of same family, family friend, neighbour. So, while they seemingly grew up "apart" in the studies, some of them grew up in the same community, neighbourhood, attended the same school, and even socialised together. It would seem the technical usage of "separated" used here is not the same as how the word is used in ordinary life.
Also there are issues in how "environment" is defined. Many aspects of environmental and cultural experience - such as parental involvement in learning, educational provision, supportive environment encouraging self-confidence and motivation - are important predictors of IQ scores.
However, when assessing relative importance of genes and environment heritability studies look only at limited range of environmental factors. One of these studies, as an example, considered things like the availability of household facilities such as an "unabridged dictionary" and a "telescope" and used these as a measure of "cultural and intellectual resources" in the home. These crude measures tell us nothing about his the resources were used and also it remains unclear why having a telescope, for example, would enhance the abilities measured in IQ tests.
All of these examples of limitations of heritability studies result in some of the variability in IQ which could be due to environmental influence being apportioned to genes, inflating the estimates about heritability of intelligence.
Personally, I feel it's both, and it's incredibly hard to determine how much came from heritability and how much is environment.