Since in this debate we are arguing about which “on average” has a more significant influence, nature or nurture, I would suggest looking at studies of monozygotic twins (MZ twins share around 100% of their genes) that were separated in infancy and adoption studies (where the environment is shared but no genes are shared) that investigate mentally and physically healthy individuals which are representative of the general population.
Minnesota Study of Twin Reared Apart (MISTRA)
During MISTRA, several hundred sets of monozygotic twins that had been separated in infancy were reunited and tested on multiple intelligence and personality tests. It was found that there was a correlation of 0.69 between the Wechsler IQ scores of MZ twins reared-apart compared to 0.88 for MZ twins reared-together and a correlation of 0.78 between reared-apart MZ twins compared to 0.76 for reared-together MZ twins on the very heavily g-loaded Raven’s Progressive Matrices test with a vocabulary scale score added to it. It was therefore concluded that about 70% in the variance in IQ-scores is associated with nature (genes) which would leave a maximum influence of 30% for nurture (Bouchard et al., 1990).
Texas Adoption Project:
During the Texas Adoption Project, biological mothers that gave their new-born babies up for adoption were adopted by adoptive families that had at least one own child and everyone involved subsequently took IQ-tests (the children took them after they had grown up). It was found that there was only a 0.1 correlation between the IQ-scores of adoptive parents and adoptive children (which shared the same family environment) compared to a 0.3 correlation between the IQ-scores of adoptive children and their biological mothers (with whom the kids shared 50% of their genes), furthermore, it was found that there is a 0.3 correlation between the IQ-scores of biologically related children in the adoptive families compared to no correlation at all between biologically unrelated children in adoptive families. This shows that the family environment plays a marginal role in determining intelligence while the genes play a much more significant role (Loehling, Horn and Willerman, 1997).
Studies on identical twins have found that 70% of the variance in IQ scores is accounted for by genes and adoption studies have found that adopted children are more similar in intelligence level to a birth mother they had never met before but shared 50% of their genes with than to an adoptive mother with whom they had spend their lives with which suggests that the family environment plays a marginal role in determining intelligence. Furthermore, the intelligence of children that are biologically unrelated but spend their whole life in the same family is not correlated at all. Therefore, nature seems to be playing a larger role than nurture when both physically and mentally healthy individuals are investigated.
Bouchard Jr, Thomas & T. Lykken, David & Mcgue, Matt & Segal, Nancy & Tellegen, Auke. (1990). Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart. Science (New York, N.Y.). 250. 223-8. 10.1126/science.2218526.
Loehlin, J. C., Horn, J. M., & Willerman, L. (1997). Heredity, environment and IQ in the Texas Adoption Project. In R. J. Sternberg & E. L. Grigorenko (Eds.), Intelligence, heredity, and environment (pp. 105-125). New York, NY, US: Cambridge University Press.