Animal Behavior/Genetic Effects on Cognitive Ability
Human Cognitive Development 
Nature, Nurture, and Cognitive Development from 1 to 16 years: In a Parent-Offspring Adoption Study, Robert Plomin and his fellow researchers conclude that genetics, not environment, play a larger role in the development of the cognitive processes. These researchers are interested in cognitive ability (the mental process of knowing, including aspects such as awareness, perception, reasoning, and judgment). Results for children aged 1-16 show that adoptees, although resembling their adoptive parents minimally at first, become more and more like their biological parents and less like their adoptive parents as they age. By adolescence, there is a strong resemblance to the cognitive functioning of the biological parents, to the same degree as in the control families, and no similarity to that of the adoptive parents. Despite the fact that the children do not spend any time with their biological parents, they are more similar, cognitively speaking. It suggests that the genes affecting cognitive ability are not all expressed until adolescence and, further, that whatever environmental factors may affect cognitive functioning are not at all correlated with the cognitive ability of the parents who raise the child. The research team conducted their 20-year longitudinal Colorado Adoption Project (CAP) in Denver, CO beginning in 1975. The CAP followed 245 adopted children and their biological and adoptive parents, as well as 245 matched nonadoptive children and their parents. They recruited pregnant mothers prior to labor. A fifth of the biological fathers were tested, whereas previous studies had not tested the biological fathers. The children were placed in homes at twenty-nine days, and in adopted homes at seven months. The researchers also had a control group of families. The children were matched to adopted families relatively similar to their own based on gender, the number of children in the family, age of the father, occupational status of the father, and father’s years of education. Also, the variability in socioeconomic status was representative of the U.S. population. The biological mothers were usually tested during their third trimester and the adoptive parents and control parents were usually tested during the child’s first year. Collectively, the thirteen tests lasted three hours long and tested the cognitive abilities of each individual. The tests were given periodically to the children throughout their development and maturation. CAP also presents a disclaimer stating that they do not believe that their examinations are fully accurate due to exceptions that they did not test, such as mental retardation. They also only represent the middle ninety percent of the U.S. population at the time in which their study began (1975). This study shows that genetic effects on cognitive abilities are not manifested only in adulthood, but also to a considerable extent in adolescence to a lesser degree in childhood, and only slightly during infancy. The different degrees of manifestation attest to the fact that not all genes regarding cognitive ability come arise until the time of adolescence. It also shows that genetics, not environment, contribute to a individuals abilities.
- Plomin R, Fulker DW, Corley R, DeFries JC. 1997. Nature, nurture and cognitive development from 1 to 16 years: A parent-offspring adoption study. Psychological Science, 8, 442-447.