Twins: early mental development.

Mental development was appraised periodically for infant twins, and the twins displayed high within-pair concordance for level of mental development during the first and second years. Twins were also concordant for the spurts and lags in development in this period (monozygotic twins more so than dizygotic). From these results it was inferred that infant mental development was primarily determined by the twins' genetic blueprint and that, except in unusual cases, other factors served mainly a supportive function.