of the individual (i.e., ontogeny) and the proximate mechanism (e.g., brain anatomy and hormones). Sociobiologists are interested in how behavior can be explained logically as a result of selective pressures in the history of a species. Thus, they are often interested in instinctive, or intuitive behavior, and in explaining the similarities, rather than the differences, between cultures. For example, mothers within many species of mammals – including humans – are very protective of their offspring. Sociobiologists reason that this protective behavior likely evolved over time because it helped the offspring of the individuals which had the characteristic to survive. This parental protection would increase in frequency in the population. The social behavior is believed to have evolved in a fashion similar to other types of nonbehavioral adaptations, such as a coat of fur, or the sense of smell. Individual genetic advantage fails to explain certain social behaviors as a result of gene-centred selection. E.O. Wilson argued that evolution may also act upon groups. The mechanisms responsible for group selection employ paradigms and population statistics borrowed from evolutionary game theory. Altruism is defined as "a concern for the welfare of others". If altruism is genetically determined, then altruistic individuals must reproduce their own altruistic genetic traits for altruism to survive, but when altruists lavish their resources on non-altruists at the expense of their own kind, the altruists tend to die out and the others tend to increase. An extreme example is a soldier losing his life trying to help a fellow soldier. This example raises the question of how altruistic genes can be passed on if this soldier dies without having any children. Within sociobiology, a social behavior is first explained as a sociobiological hypothesis by finding an evolutionarily stable strategy that matches the observed behavior. Stability of a strategy can be difficult to prove, but usually, it will predict gene frequencies. The hypothesis can be supported by establishing a correlation between the gene frequencies predicted by the strategy, and those expressed in a population. Altruism between social insects and littermates has been explained in such a way. Altruistic behavior, behavior that increases the reproductive fitness of others at the apparent expense of the altruist, in some animals has been correlated to the degree of genome shared between altruistic individuals. A quantitative description of infanticide by male harem-mating animals when the alpha male is displaced as well as rodent female infanticide