The murder of young children, either individually or en masse, is usually a result of social attitudes. Infanticide has historically been associated with the birth of a deformed or abnormal child, of twins, or of a female child. The major contributing factor is generally poverty, although illegitimacy also plays a large role, especially in more contemporary societies. Infanticide also occurs during movements of people – warrior societies in the past, and refugee movements in recent times – and as a means of evading governmental policies such as China's "one child per family". Instead of outright infanticide, the child may be abandoned (though few are found alive), while other cases take the form of ritual sacrifices.
Infanticide has been a common practice since earliest prehistoric times among pagan, primitive tribes throughout the world. An example of the latter is the Jagas tribe of west Africa which killed all its own children, maintaining the population by adopting the children of its victims. Infanticide was strongly recommended by both Plato and Aristotle as a means of ensuing the stability of the population of Athens. In traditional Japanese culture, the term used was "thinning the family", whereby daughters were not killed at birth but were allowed to grow to an age when their strength or intelligence became apparent (2 to 4 years). The weaker were then drowned or decapitated. Infanticide was extensively practised in ancient China, especially in times of famine. It was a common practice in Rome where the father had the right to dispose of unwanted children by throwing them in the Tiber.
Examples of infanticide can be found in all societies, both past and present, ranging from isolated individual instances to general policy. Much attention has been focused on the practice in modern China where the number of infanticides (generally females) may exceed 10,000 per year, according to the USA National Academy of Sciences.