The general body shapes of female and male bodies both have significant social and cultural symbolism. Physical attractiveness is closely associated with traits that are considered typical of either sex. The body mass index ( BMI ), waist-to-hip ratio, and especially waist-to-chest ratio in men have been shown in studies to rank as overall more desirable to women. Wide shoulders, a muscular upper body and well-developed upper-arm muscles are all traits closely associated with masculinity , similarly to other specifics of the male sex, like beards . These traits are seen more sexually attractive to women and also associated with higher intelligence, good leadership qualities and better health. 
Most mammals , including humans , are genetically determined as such by the XY sex-determination system where males have an XY (as opposed to XX) sex chromosome . It is also possible in a variety of species, including humans, to be XXY or have other intersex / hermaphroditic qualities, though one would still be considered genotypically (if not necessarily phenotypically) male so long as one has a Y-chromosome. During reproduction , a male can give either an X sperm or a Y sperm, while a female can only give an X egg. A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female .