Minor physical anomalies (MPAs) are an index of deviant embryological development due to genetic defects of insults to the fetus. A brief 10-minute examination an an individual makes it possible to establish a count that shown highly stable individual differences from the newborn period up to age seven years, the latest age studied longitudinally. For males, high MPA counts in the newborn period have shown strong predictive relationships to preschool temperament factors such as short attention span, high activity level, and aggressive-impulsive behaviour. For females, high anomaly scores showed relationships to short attention span and to inhibition. Such findings are in contrast with inconsistent results from genetic studies and with infrequent, weak relationships of neonatal variables to later behaviour. These results from the Bethesda longitudinal study have been confirmed in most cases by several cross-sectional studies. Sex differences in behaviour, usually considered to result from differential sex-role training, disappear in preschool samples from which high MPA individuals have been removed, but are much more pronounced than usual in samples with high MPAs.