Polygyny is much more common than polyandry because evolutionarily, the benefits of polygyny for men are much higher than the benefits of polyandry for women. This is true for reasons outlined in my previous post; in short, men benefit reproductively much more than women do from having multiple partners, and so they are relatively psychologically adapted to seek novel mates. By the same token, the evolutionary costs of polyandry for men are much higher than the costs of polygyny for women. If you're a man whose co-husband has impregnated your shared wife, then you have to wait a long time (at least nine months of gestation, and probably years of lactation as well) before you get another shot at reproduction. But if you're a woman whose co-wife has been impregnated by your shared husband, you can still get pregnant right away. You'll have to share your husband's parental investment with your co-wife, but at least you'll be able to reproduce. And being the co-wife of a wealthy, high status, highly desirable man may well be a better option for you than being the only wife of a less impressive husband. Although some Westerners will wonder why any woman would choose to marry polygynously rather than monogamously, many women cross-culturally nevertheless do choose to do just that.