Sexual relations and reproductive behavior are plainly a central part of the crowding equation. Conceived in an objective sense, household crowding is simply a function of too many people occupying too small a space. The ethological literature suggests that under conditions of extreme crowding, adult animals do not care for their young as effectively and higher levels of mortality result. The chapter explores these issues by examining patterns in child mortality, miscarriages, and abortion. Most of what is known about crowding and sexual behavior is derived from ethological research. Observations of crowded animals have suggested that such animals exhibit aberrant forms of sexual behavior, aberrant at least in terms of the respective patterns normally observed in those animal populations. The evidence concerning the effects of crowding on the reproductive behavior of animals is somewhat more consistent, generally reporting an inverse relationship.