Humans were all once hermaphrodites of sorts. Our sexual configuration is the product of sex chromosomes passed on by our parents. Because females have only X sex chromosomes, we can receive only an X from our mothers; we can receive an X or a Y from our fathers because males carry both kinds. If we get two Xs, we become XX genotypical females; if we get an X and a Y, we become XY genotypical males. It is the Y chromosome that determines human sex. The "basic" human is female; it is not the presence of two Xs that makes a female, but rather the absence of the Y chromosome. Sometimes children are born with several X chromosomes, such as the XXX, XXY, or even XXXXY patterns. Although the X chromosome carries far more genetic information than the Y, no matter how many X "units" of femaleness the fetus has, the presence of a single Y results in the male pattern of sexual differentiation. The extra Xs, however, do affect development, since it is characterized by hypogonadism As discussed below, individuals with the extra Xs are classified as having Klinefelter syndrome. The single exception to this male differentiation by possessors of a Y chromosome is the XY genotype with androgen-insensitivity syndrome.
http://www2.hu-berlin.de/sexology/GESUND/ARCHIV/SEN/CH12.HTM#b2-HERMAPHRODITES
In 1942, Dr. Harry Klinefelter and his coworkers at the Massachusetts General Hospital in Boston published a report about nine men who had enlarged breasts, sparse facial and body hair, small testes, and an inability to produce sperm.
By the late 1950s, researchers discovered that men with Klinefelter syndrome, as this group of symptoms came to be called, had an extra sex chromosome, XXY instead of the usual male arrangement, XY.
http://www.nichd.nih.gov/publications/pubs/klinefelter.htm
