What the Aka and Ngandu have in common, besides geography, is this: In both cultures, men and women view sexual intercourse as a kind of "work of the night." The purpose of this work is the production of children -- a critical matter in an area with a very high infant mortality rate. Semen is understood by the Aka and Ngandu to be necessary not only to conception, but also to fetal development. A woman who is already pregnant will see having intercourse as contributing to the health of her fetus.

The Aka and Ngandu speak of sex as "searching for children." That's not to say they don't enjoy having sex. Clearly they do. The Hewletts relay a song a group of children invented after stealthily watching two lovers having sex. In the song, the man asks, "How do you want it?" and the woman answers, "Oh, I want it big." The man asks again, and the woman answers, "Oh, I want it long." The song then enters a refrain with the man thrusting and asking his partner, "Did you come?"

But while the individuals the Hewletts interviewed -- like the song -- made it clear that sex is pleasurable for these folks, and something that brings couples closer, they also made clear that babies are the goal of sex. Said one Aka woman, "It is fun to have sex, but it is to look for a child." Meanwhile, a Ngandu woman confessed, "after losing so many infants I lost courage to have sex."

Is the strong cultural focus on sex as a reproductive tool the reason masturbation and homosexual practices seem to be virtually unknown among the Aka and Ngandu? That isn't clear. But the Hewletts did find that their informants -- whom they knew well from years of field work -- "were not aware of these practices, did not have terms for them," and, in the case of the Aka, had a hard time even understanding about what the researchers were asking when they asked about homosexual behaviors.

The Ngandu "were familiar with the concept" of homosexual behavior, "but no word existed for it and they said they did not know of any such relationships in or around the village. Men who had traveled to the capital, Bangui, said it existed in the city and was called 'PD' (French for par derriere or from behind)."

Given all this, the Hewletts conclude, "Homosexuality and masturbation are rare or nonexistent [in these two cultures], not because they are frowned upon or punished, but because they are not part of the cultural models of sexuality in either ethnic group."

The finding with regard to homosexuality is perhaps not that surprising. As the Hewletts note, other researchers have documented cultures where homosexuality appears not to exist. If homosexual orientation has a genetic component to it -- and there is increasing evidence that it does, in many cases -- then it would not be surprising that this complex human trait (one that involves non-procreative efforts) would be found in some populations but not others.