There are some indications from animal experiments that population density in itself may affect mental health. Thus, under conditions of extreme overcrowding, an increase in male aggressiveness and an accompanying decline in the adequacy of maternal behaviour have been observed among rats. When the young were methodically removed from colonies of rats kept at high densities, it was noted in addition that bands of young males assaulted females and that there was an increase in homosexual behaviour. However, mice brought up under overcrowded conditions showed less stress behaviour than those transferred only later to such conditions. Some epidemiological studies have shown higher rates of schizophrenia, crime, suicide, alcoholism, and drug abuse in the central, more crowded areas of old established cities than in other areas of such cities, but correlations of this kind are by no means clear-cut.
El Salvador has been the most densely populated country in the Western Hemisphere, with about 243 people per square kilometre. The problem of population density was particularly serious in the San Salvador urban area. According to a 1995 study, the urban area had 30 percent of the nation's population, with a density more than twice that of the nation as a whole.