Argument by way of regression towards the mean

In his review of evidence, Nisbitt (2009) notes that regression towards the mean studies provide evidence, albeit weak, for the genetic hypothesis. He contends that the evidence is weak because the same findings can be explained environmentally. He and other nonetheless, have yet to offer a compelling explanation for them.

With regard to those studies, it has been found that both Black and White siblings (e.g., Jensen, 1973 & Murray, 1999) regress towards (up and down) their respective IQ means. Now it’s a mathematical given that two groups drawn from a common population will show regression, but unless there is some differentiating factor with respect to the dimension measured, they will regress to a common mean. If two groups regress towards separate population means, a causal explanation is wanting. Theoretically environmentalists can account for this. The specific mechanism they propose, however, must account for the following:

a) Why siblings of relatively high IQ Blacks and Whites show regression downwards to separate means.
b) Why siblings of relatively low IQ Blacks and Whites show regression upwards to separate means.
c) Why this pattern of regression is fairly constant across generations, social class, the IQ spectrum, and US geography. [Murray’s data comes from the nationally representative NLSY (80’s to 90’s); Jensen’s data comes from a California school district (60’s)]
d) Why the magnitude of the regression is what it is, that is, roughly as predicted by an additive genetic model.

Murray, 1999. The Secular Increase in IQ and Longitudinal Changes in the Magnitude of the Black-White Difference: Evidence from the NLSY

References

Jensen, 1973. Educability and group differences. P. 114-119.

Murray, 1999. The Secular Increase in IQ and Longitudinal Changes in the Magnitude of the Black-White Difference: Evidence from the NLSY