Social and play responses of first-born and later-born infants in an unfamiliar situation.
D001722:Birth Order; D004777:Environment; D005106:Exploratory Behavior; D005149:Facial Expression; D005239:Fear; D005260:Female; D006801:Humans; D007223:Infant; D007398:Interpersonal Relations; D008297:Male; D010988:Play and Playthings; D012932:Social Facilitation