Eh? Interesting, about Russian birth rates, but so what? We're talking about why nature might arm one with certain sexual instincts, which would be to help us survival in small, isolated tribes on a veldt.
But, let me point out, that instinct may be operating accurately even yet: have you ever seen a mother cat kill and eat the runt of her litter? Did you know that solitary mammal predators will frequently eat all the offspring of a litter if times seem particularly bad just then? Do you want to claim that these are bad investment strategies, genetic survival-wise? If you do, I would disagree: survival is as much about securing resources--and in humans, that could even mean philosophical/emotional/social resources as much as meat on the table--as it is about reproduction.
Cheers!