Peter H. Schönemann
Professor Emeritus • Department of Psychological Sciences • Purdue University
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Note: numbers in brackets refer to Publications list.

Quantitative Behavior Genetics

One reason for the astonishing persistence of the IQ myth in the facce of overwhelming prior and posterior odds against it may be the unbroken chain of excessive "heritability" claims for "intelligence", which IQ tests are supposed to "measure". However, if  "intelligence" is undefined, and Spearman's g is beset with numerous problems, not the least of which is universal (and by now tacitly though grudgingly acknowledged) rejection of Spearman's model by the data, then how can the heritability of "intelligence" exceed that of milk production of cows and egg production of hens?

 

These problems are addressed in a series of more recent publications, [54, 60, 61, 62, 63, 70, 71, 72, 75, 81]. In [70] it is shown that a once widely used "heritability estimate" is mathematically unsound, because Holzinger had made a mistake in his derivations which had been overlooked for decades. Another such estimate, though mathematically valid, never fits any real data. This should have been obvious from the start because it typically produces an inordinate number of  inadmissible estimates (e.g., proportions larger than 1). These absurd results nevertheless found their way into print without comment or challenge. The same estimate also produces excessive "heritabilities" for variables which plainly have nothing to do with genes. For example, the "heritability" of answers to the question: "Did you have your back rubbed last year?" turns out to be 92% for males and 21% for females [81].

 

The main problem is that all such estimates rely on  simplistic mathematical models which necessarily make some unrealistically stringent assumptions. Unfortunately, they were rarely tested. Once they are tested, one finds that they are usually violated by the data. A comprehensive review of these issues is attempted in [81], where further references to specific subproblems can be found.