With changes in sex role expectations in marriage, family researchers have begun to examine the concept of machismo. Two characteristics dominant in the study of machismo are aggressiveness and hypersexuality. A biological model of machismo asserts that males everywhere tend to be more aggressive than females, a sex difference which appears to have a genetic base. A modern theory of sociobiology offers another explanation for macho behavior. According to this theory, much of animal, and perhaps human, behavior is influenced by the drive for one's genes to reproduce themselves. A generally accepted psychological theory views machismo as an expression of an inferiority complex. Most research on machismo is restricted to the lower classes. Research from Mexico, Puerto Rico, England, and the United States suggests that lower class males suffer from job insecurity and compensate for their feelings of inferiority by exaggerating their masculinity and by subordinating women. Other studies point to distant father-son relationships as one factor leading to feelings of inferiority and to the development of machismo. Women may support machismo by being submissive, dependent, and passive. The combination of feeling inferior and acting superior is machismo, a trait that is repeated generation after generation. If men can be socialized toward male parental investment, the incidence of machismo may decline and the incidences of men feeling self-esteem and women feeling equal to men may rise. (NRB)