Temperament and craniofacial variation in the first two years.

High levels of circulating glucocorticoids in fetal animals are associated with inhibited development of the lateral processes of the maxilla, or upper jaw, as these bones grow toward midline union early in gestation. This inhibition can prevent fusion and lead to the formation of cleft palate. Mouse strains are differentially susceptible to this effect. These strains also differ in behavioral and physiological characteristics that are analogous to those we have observed in 2 temperamental groups of children, analogies that prompted the current study. Infant temperament was assessed using behavioral observation at 4 months, and the ratio of upper facial width (bizygomatic breadth across the cheekbones) to height of the head and face was measured at 14 and 21 months. High reactive 4-month-old infants, who are predisposed to becoming timid, inhibited toddlers, had smaller bizygomatic ratios (narrower faces) at both 14 and 21 months compared to their low reactive peers, who are predisposed to becoming outgoing, uninhibited children. Differences in facial width were modest though significant and not attributable to overall stature.