Eventually, they learn the truth. A plague wiped out most human life, including all males. Only about 11,000 women survived, mostly concentrated in Australasia and a few other areas. Until recently, they reproduced only by cloning, so most women are clones of the original 11,000 genotypes. Babies are raised communally in crèches, and all members of each genotype are encouraged to add their story to a book that is passed on for the inspiration and education of future "sisters." Certain genotypes are given early androgen treatments (hence, Andy, who the astronauts thought was male) to increase physical tasks' bulk and strength.