Family and parenting characteristics are not significant contributors to variation in IQ scores, Beaver et al. find, suggesting that IQ is in the genes:
To find out, the team pored over information from a study of more than 15,000 U.S. middle- and high-school students. It’s called the National Longitudinal Study of Adolescent Health.
Starting in the 1994-to-1995 school year, researchers had asked students a series of questions. For instance: How warm and loving are your parents? How much do you talk with them? How close do you feel to your parents? How much do you think they care about you?
Students also were given a list of 10 activities. Then the questionnaire asked how many of those activities students had done with their parents in the previous week. Did they play sports together? Go shopping? Talk with each other over dinner? Watch a movie together?
Students also answered questions about how permissive their parents were. For example, did their parents let them choose their own friends, choose what to watch on TV or choose for themselves when to go to bed?
The researchers then gave the students a test to gauge their IQ. Called a Picture Vocabulary Test, it asked the students to link words and images. Scores on this test have been linked repeatedly to IQ. Later in life, between the ages of 18 and 26, these people were tested again.
Beaver’s group was especially interested in results from a group of about 220 students who had been adopted. The parents who raised them had not passed on any genes to them. So if there was a link between the students’ IQs and the way their parents raised them, the researchers should see it most clearly in the adopted students’ scores.
But no such link emerged. Whether students reported their parents cared about them and did things with them — or reported that they did not — it had no impact on the their IQ.
Full text: A closer look at the role of parenting-related influences on verbal intelligence over the life course: Results from an adoption-based research design.