  Paternal age at conception has been found to predict the number of new
genetic mutations. We examined the effect of father's age at birth on offspring
intelligence, head circumference and personality traits. Using the Minnesota
Twin Family Study sample we tested paternal age effects while controlling for
parents' trait levels measured with the same precision as offspring's. From
evolutionary genetic considerations we predicted a negative effect of paternal
age on offspring intelligence, but not on other traits. Controlling for
parental IQ had the effect of turning a positive-zero order association
negative. We found paternal age effects on offspring IQ and MPQ Absorption, but
they were not robustly significant, nor replicable with additional covariates.
No other noteworthy effects were found. Parents' intelligence and personality
correlated with their ages at twin birth, which may have obscured a small
negative effect of advanced paternal age (< 1% of variance explained) on
intelligence. We discuss future avenues for studies of paternal age effects and
suggest that stronger research designs are needed to rule out confounding
factors involving birth order and the Flynn effect.
