The present invention provides a phenotypically stable altered animal which exhibits unprovoked rage reaction indicative of high emotionality, stereotyped behavior and is useful as a model of psychotic behavior. The animals of this invention (F.sub.2) were obtained by inbreeding mature offspring (F.sub.1) of rats whose mothers received high tryptophan diets during their third trimester of gestation. There is also an increased body weight which evidences physical alteration. The present invention also relates to the preparation and use of animal models for the study of psychosis, pathological psychotic behavior in humans and the effects of tryptophan in successive generations. The prenatal effects of tryptophan on later generations can also be used to increase the body weight of experimental animals and livestock.