Biological factors and psychometric intelligence: a review.

Area of Science:

• Behavioral genetics
• Human physical and cognitive traits

Background:

• Investigating the relationship between psychometric intelligence and physical traits like head and body size is crucial for understanding genetic influences.
• Previous research has explored pleiotropy and prenatal effects in relation to these correlations, with mixed findings.

Purpose of the Study:

• To synthesize correlations between psychometric intelligence and physical traits (head size, body size).
• To examine evidence for pleiotropy between intelligence and physical traits.
• To assess the impact of prenatal effects on twin study heritability estimates.

Main Methods:

• Meta-analysis of existing studies on intelligence, head size, and body size.
• Review of within-family studies to assess pleiotropy.
• Analysis of studies on prenatal effects and their influence on twin studies.

Main Results:

• A mean partial correlation of .10 (controlling for height) was found between intelligence and head size.
• Estimated correlation between intelligence and brain size was .29 (all studies) and .44 (IQ tests).
• Partial correlations (controlling for age) between intelligence and height were .18 (children) and .22 (adults).
• Within-family studies suggest pleiotropy contributes to intelligence correlations with head and body size.
• Prenatal effects were found to be negligible bias in twin studies.

Conclusions:

• Psychometric intelligence is moderately correlated with head and body size, potentially due to pleiotropy.
• Twin study heritability estimates are not significantly biased by prenatal effects.