Variety is the Spice of Life Scholars have written tomes on the subject of variation in animal approaches to sex/reproduction. Although scientists have studied socially determined sex change in fish since the early 1970s, there is a lot we don’t know about how it works. The “dazzling array” of approaches to sexual reproduction found in vertebrates is calledprimary sex determination. Many vertebrates-including humans-use chromosomal sex determination: a heritable genetic element attached to a chromosome usually directs development down one of two pathways. The phrase “sex determination” suggests that one is talking about both male developmentand female development. Fetal hormonal sex, in turn, plays a critical role in the development of secondary sex determination-the differentiation of male or female internal organs and, in due time, the differentiation of the external genitalia. Even though the fetal gonad produces androgens, the cells cannot capture the androgen molecules and thus cannot use them to move development in a male direction.