Q. A friend told me recently that males and females begin life with both sets of sex organs and that each possess the same hormones. That’s probably just another one of those theories to justify bisexuality. Have you ever heard of such rubbish?

A. Rubbish as applied to theories to justify bisexuality or to the perception that human beings begin life with both sets of sex organs and that each possesses the same hormones? Personally, I’ve never thought of either hormones or sex organs in terms of rubbish.

Since every brain on the planet is believed to differ in structure, function, and perspective, each brain has its own opinion. The tone of your question suggests that your opinion on the topic of sex organs and hormones differs rather dramatically from that of your friend. The brain and body are “fearfully and wonderfully” constructed. Whatever else humans are (e.g., relational, spiritual), they are sexual beings at their very core. That does not indicate, however, that a person’s core sexual being and the sexual behaviors he/she chooses to exhibit are one and the same.

In terms of sets of sex organs, your friend was on the right track. Here is a brief summary.

Internal sex organs: Both genetically male (XY) and genetically female (XX) fetuses start out with two sets of internal primordial structures, the Wolffian and the Mullerian.

• In the presence of testicular hormones, the Wolffian ducts develop (e.g., prostate and vas) and the Mullerian ducts regress.

• In the absence of testicular hormones the reverse happens. The Wolffian ducts regress and the Mullerian ducts develop (uterus and fallopian tubes).

External sex organs: Both male and female fetuses also start out with a single set of external primordial structures:

• Testosterone stimulates these structures to differentiate into penis and scrota, becoming recognizably male by about week 9-10 of gestation.

• In the absence of testosterone, these same structures become clitoris and labia, regardless of the levels of estrogen or progesterone. So, no hormonal influence from the female gonads (ovaries) appears to be needed for differentiation of female external genitalia. Some have referred to this as a preprogrammed state, perhaps contributing to the perception that the default position for a human fetus is female.

Your friend was also on the right track in relation to the topic of hormones. As members of the same species, males and females are far more alike than they are different. This means that as far as is presently known, human beings have the same types of hormones. Relative hormonal levels differ, however. Males tend to have estrogens but at much lower levels than those generally found in females. Females have testosterone but at a much lower level than that typically found in males. Some studies have shown that at puberty the average male has 20 times the testosterone in a comparable female. And speaking of testosterone, competition appears to increase the level of testosterone in the male. Interestingly enough, competition doesn’t appear to have much impact on testosterone levels in the average female.

Sex organs and hormones as rubbish? Not so much. Complexity on top of complexity and fraught with potential for variation? You bet!