Two facts always need to be borne in mind regarding polygyny: First, that wherever it is practiced it is relatively confined to the upper and wealthy classes, for the reason that the support of more than one family is something which only the wealthy classes in a given society could assume. Secondly, it follows that under ordinary circumstances only a small minority of a given population practice polygyny, even in countries in which it is sanctioned. In Mohammedan countries like Turkey and Egypt, for example, it is estimated that not more than five per cent of the families are polygynous, while in other regions the percentage seems to be still smaller. The reason for this is not only the economic one just mentioned, but that everywhere the sexes are relatively equal in numbers, and therefore it is impossible for polygyny to become a widespread general custom. If some men have more than one wife it is evident that other men will probably have to forego marriage entirely. This is not saying that under certain circumstances, namely, the importation of large numbers of women, a higher per cent of polygynous families may not exist. It is said that among the negroes on the west coast of Africa the number of polygynous families reaches as high as fifty per cent, owing to the fact that female slaves are largely imported into that district, and that they serve not only as wives, but do the bulk of the agricultural labor, the male negro preferring female slaves, who can do his work and be wives at the same time, to male slaves. But such cases as these are altogether exceptional and manifestly could not become general. Summing up, we may say that the causes of polygyny are, then: (1) First of all, the brutal lust of man. No doubt man's animal propensities have had much to do with the existence of this form of the family. Nevertheless, while male sensuality is at the basis of polygyny, it would be a mistake to think that sensuality is an adequate explanation in all cases. On the contrary, we find many other causes, chiefly, perhaps, economic, operating also to favor the development of polygyny. (2) One of these is wife capture, as we have already seen. The captured women in war were held as trophies and slaves, and later became wives or concubines. Among all peoples at a certain stage the honor of wife capture has alone been a prolific cause of polygyny. (3) Another cause, after slavery became developed, was the high value set on women as laborers. Among many barbarous peoples the women do the main part of the work. They are more tractable as slaves, and consequently a high value is set upon their labor. As we have already seen, these female slaves usually serve at the same time as concubines, if not legal wives of their masters.