According to the calculations being made in various places in Europe, more boys are born there than girls  ; contrariwise, the relations of Asia tell us that many more girls are born there than boys.  The law of a single wife in Europe and the one which that allows several in Asia therefore have a certain relationship to the climate.
But I have difficulty believing there are many countries where the disproportion is great enough to require the introduction of the law of multiple wifes or the law of multiple husbands. That just means that the plurality of women or even the plurality of men is more consonant with nature in certain countries than in others.
I admit that if it were true, as the relation tell us, that in Bantam there are ten women for every man,  that would be a most particular case of polygamy.
In all this I am not justifying the customs, but giving the reasons for them.