Nearly 35 years ago, as an undergraduate, I took a course in "Differential Psychology." The course in "individual vs. group differences" was essentially a "nature vs. nuture" course as applied to mental traits. We studied the contribution of genetics vs. environment for a wide variety of things including schizoprhenia, other pyschopathology, addiction, criminality and just extroversion vs. introversion. However, nearly half the semester involved the studying the comparative influence of genetics vs. environment for intelligence and I found that discussion particularly fascinating.
We reviewed a wide number of statistical studies evaluating the question. Many of the studies involved identical twins raised apart from each other. Because they are identical genetically presumably any differences in their intelligence would be attributable to environment while similarities beyond those of the general population should be attributable to genetics.
What we reviewed in the late 70s indicated that genetics had a strong contribution to intelligence but there was also an environmental component. Even so, it appeared that the genetic contribution might be as high as 80% (which means environment could still move intelligence by around a standard deviation, both up and down, creating two standard deviations of variance from a poor vs. enhancing environment).
To say the least, the research has moved on. Scientists are now identifying the specific genes associated with high IQ.
It will only be a matter of time before parents can test for the presence of these desirable high IQ genes in utero. Or to take it even further, they could artificially fertilize any number of "bench embyros" and then pick only those containing the most desirable high IQ genes to inseminate.
"The world is a mess and I just need to rule it." --Dr. Horrible.