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. 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 cleft between the buttocks , which according to Morris in The Naked Ape is also unique to humans, other primates as a rule having much flatter buttocks. 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 paraphilia, but, in modern times, this attraction is considered normal unless it is highly atypical and is therefore a form of partialism.