Dominant females leave more surviving young on average than subordinate ones. Women, Hrdy argues, still have every incentive to be ambitious - ambition to gain social status is in their genes. This does not mean that every woman is ambitious all the time. No lineage has ever been perpetually dominant, or not for long enough to breed a line of blue-blooded snobs with secure places at the top of the female hierarchy. What has been bred in is the ability to work the social system - knowing how to deal with life as a subordinate, and when ambition will pay off in making a bid for social power.
Mammalian females go for quality offspring, more than quantity.
If you simply count genes, the bottom line is having great-grandchildren, not how big your immediate family is. Two children who do well enough to raise offspring of their own are a better bet than many children, all of them at risk. The odds have changed, of course, in modern society, but the body's calculations of how many children it can afford to bear run beneath the calculations of the conscious mind. It often means concentrating all your efforts as a mother on just a few. Whether the limit is imposed by delayed oestrus, sexual abstinence, birth control, spontaneous or induced abortion, or at an extreme, abandoning a baby when there is no hope of raising it, mothers make such choices.