> The most elegant proof of IQ being linked to genetics: > > The same person taking an IQ test twice has mean correlation of 0.85 or above in their scores. Identical twins reared together: 0.86 Identical twins reared apart: 0.76 Biological parent and child: 0.42 Adoptive parent and child: 0.19 And of course, any two random people will have a correlation close to 0.

How is this proof of IQ being linked to genetics if adoptive parents can have 1/2 of the correlation that identical twins have? I think this proves that genetics only has a minor part, and upbringing/environment seems to be the most important factor.

First of all, you misread the figures. The correlation between identical twins reared together is 0.85, while adoptive parent and child is 0.19.

Second of all, correlation coefficients do not scale linearly. A correlation of r=0.85 means r^2 explains 0.72 of the variance, while a correlation of 0.19 explains 0.03 of the variance. In other words, adoptive parents have 0.03/0.72=4% of the effect on IQ as being an identical twin does.