As we move into the twenty-first century, the effects of human crowding loom as an ever larger and more pressing issue. In the next ten years, it is estimated that 1.1 billion people will be added to a world population already exceeding 5 billion. Much of this increase will take place in the less developed regions of the world, where the provision of housing is especially problematic. The result will likely be cities of ever higher density, with more intense crowding at the household level. How does household crowding affect human behavior? Ethological studies have documented numerous and often extreme aberrations among lower animals. However, sociological and psychological investigations of humans have produced very mixed results. North American and European studies, in particular, have generally suggested that the consequences of crowding are highly selective and mild. Prior research has relied mainly on surveys of cities with limited crowding. Here, an international team of researchers examines the impact of household congestion in a context more typical of the developing world. Their findings are based on a large representative sample of Bangkok, Thailand, where the average number of persons per room is over four times that found in North America. Relying on both qualitative and quantitative data, the analyses reported are far-reaching and include an examination of psychological well-being, marital and family relations, sibling relations, violence within the family, the impact on marital sex and reproductive behavior, and the consequences for physical health. As opposed to previous findings in industrialized countries, these results show that household crowding has a wide range of detrimental social and psychological effects.