PERFECT DRUG - If you're refering to some women who get a tremendous drive to become mothers, it's partially due to biology, and partly due to the environment.
If you'd like to read a very compelling and deep theory behind all reproductive urges, I recomend the bestseller, "The Selfish Gene" by Richard Dawkins. It's written in lay terms and is easy to understand. In essence, his idea is that "it's all about the genes. It's the genes that, for their own good, are manipulating the bodies they ride about in. The individual organism is a survival machine for its genes." In other words, our intelligence, our culture, our great inventions and not so great inventions (like god) are all due to the manipulation of the body by it's genes. DNA controls us. It's taking Freud's "biology is destiny" several steps further, and it greatly expands upon Darwinian natural selection. Needless to say, anyone who hasn't read "On the Origin of the Species" should do so immediately. Perhaps it will help gain an understanding of why the "instincts" referred to by COCO arose in the first place (or second place or 20th place, etc.)
<img src="http://members.aol.com/rnfpantera/hking1">