Instinct, in biology and psychology, behaviors in reaction to external stimuli that have not been consciously learned. It is in fact difficult to separate such inherited genetic behaviors from those stemming from learned and environmental factors, since higher animals placed from birth in artificial environments display some, but not all, instinctive reactions characteristic of their species. It has been further suggested that embryos may have some learning ability—that is, that some learning before birth is possible. Numbered among the instincts are the sex drive, aggression, territoriality, and the food urge, but much debate surrounds such classification. In psychology, “instinct” (sometimes called “drive”) is studied particularly with regard to the frustration of or conflict between the existence of two fundamental instincts: the life instinct, akin to the libido, and its opposite, the death instinct.