"Studies have shown that identical twins who grow up in separate families have an IQ correlation of 0.74, while adoptive siblings have no more similar IQ than strangers."

First off the bat, such studies are obviously social science, not science.

Secondly - there is this idea that one can boil entire brains down to one number like a CRC or checksum - the IQ number. Then you can rank them in order I suppose. It is obviously a ludicrous endeavor on reflection. It's like the Douglas Adams joke that the answer to life, the universe and everything is 42. When science actually makes progress on the brain, I'm sure biologists of the future will look on IQ like we look on phrenology.

Thirdly, these social science studies of twins mentioned were done by Thomas J. Bouchard, Jr. He's someone who writes op-eds for the Wall Street Journal - I guess Nature and Science are too full and his work crowded into there.

The second edition of the Mismeasure of Man by Stephen Jay Gould is a good book on this topic.

There was a hysterical conservative reaction to Gould's book just as there was a hysterical progressive reaction to the Bell Curve. Which shows this is really a political debate, not a scientific one. This is a political debate going back about 10,000 years, really. I am skeptical of any social scientific study that proposes it has found all the answers.