Intelligence, like height, is highly heritable. In discussing the Flynn effect, Dome Improvement shares some heritability numbers:
The classic heritability research paradigm is the twin adoption study: Look at IQ scores for thousands of individuals with various forms of shared genes and environments, and hunt for correlations. This is the sort of chart you get, with 100 being a perfect match and 0 pure randomness:
The same person tested twice: 87
Identical twins raised together: 86
Identical twins raised apart: 76
Fraternal twins raised together: 55
Biological siblings: 47
Parents and children living together: 40
Parents and children living apart: 31
Adopted children living together: 0
Unrelated people living apart: 0
Identical twins raised together really are identically intelligent; their test scores correlate almost as strongly as the same person taking the test twice. And adopted children are no more alike (as far as intelligence is concerned) than random strangers.
Of course, high heritability doesn’t mean what most people think it means:
Imagine “somebody who starts out with a tiny little physiological advantage: He’s just a bit taller than his friends,” Dickens says. “That person is going to be just a bit better at basketball.” Thanks to this minor height advantage, he tends to enjoy pickup basketball games. He goes on to play in high school, where he gets excellent coaching and accumulates more experience and skill. “And that sets up a cycle that could, say, take him all the way to the NBA,” Dickens says.Now imagine this person has an identical twin raised separately. He, too, will share the height advantage, and so be more likely to find his way into the same cycle. And when some imagined basketball geneticist surveys the data at the end of that cycle, he’ll report that two identical twins raised apart share an off-the-charts ability at basketball. “If you did a genetic analysis, you’d say: Well, this guy had a gene that made him a better basketball player,” Dickens says. “But the fact is, that gene is making him 1 percent better, and the other 99 percent is that because he’s slightly taller, he got all this environmental support.” And what goes for basketball goes for intelligence: Small genetic differences get picked up and magnified in the environment, resulting in dramatically enhanced skills. “The heritability studies weren’t wrong,” Flynn says. “We just misinterpreted them.”
Matt Ridley’s Nature via Nurture explores the interplay between genes and environment in greater detail.