As a paraphilia, breast fetishism (also known as mastofact, breast partialism, or mazophilia) is a highly atypical sexual interest focused on female breasts (see partialism). The term breast fetishism is also used in the non-paraphilic sense, to refer to cultural attention to female breasts and the sexuality they represent, with debate existing as to whether the modern widespread fascination with breasts among heterosexual males in western societies is a sexual fetish.
Scientists have hypothesized that non-paraphilic sexual attraction to breasts is the result of their function as a secondary sex characteristic. For instance, zoologist and ethologist Desmond Morris theorizes that cleavage is a sexual signal that imitates the image of the , which according to Morris in The Naked Ape is also unique to humans, other primates as a rule having much flatter buttocks.Evolutionary psychologists theorize that humans' permanently enlarged breasts, in contrast to other primates' breasts, which only enlarge during ovulation, allows human females to "solicit [human] male attention and investment even when they are not really fertile".
The reverence and theorizing shown to breasts also appears in the science of modern civilization. Breast fetishism is claimed to be an example of a contagious thought (or meme) spreading throughout society, and that breasts are features that have evolved to influence human sexuality rather than serve an exclusive maternal function.
In clinical literature of the 19th century, the sexual focus on breasts was considered a form of paraphillia, but, in modern times, this attraction is considered normal unless it is highly atypical and is therefore a form of partialism.