Neonatal temperament in full-term twin pairs discordant for birth weight.

To examine the significance of relative birth weight of cotwins for neonatal temperament, 70 pairs of full-term twins who were at least 15% discordant for birth weight were assessed. The assessment focused on irritability, resistance to soothing, activity, reactivity, and reinforcement value. The larger twin of the pair was more irritable, more difficult to soothe, more active while awake, more active during sleep, less reactive to visual and auditory stimuli, and less reinforcing to the examiner than the smaller cotwin. Both the lighter and heavier cotwins exhibited behavioral discrepancies. The results are discussed in relation to prenatal influences on early behavioral development, and implications for differential parent-infant interaction with each twin.