Baresark said:
The best rate of success was greatly increased when not having more kids than you could hope to find food for and protect. Even if you could have 7 kids, you more than likely would not be able to both feed and protect the majority of them. In primitive societies, most people were also nomadic, so a woman who spent a 7-10 year chunk of her life pregnant or feeding would be to big of a burden on a small society.
That doesn't mean "permapregnancy" was not happening though, unless there are some data that back this up. Are there some decent methods to prove what was the actual number of children raised by an average family (and I mean physical sources) rather than merely a calculation that a specific number of children was beneficial? If several thousands years of more recent history are of any indication, infanticide "ad hoc" could've been much easier to figure out rather than some "natural" version of a contraception. There's an apt number of examples from behind the fence, where our various cousins live. Not sure which version of animal birth control should we presume for our ancestors though - and they vary a LOT.
Same about being a burden - it merely implies there's a cost, do we have any clue how a group defined efficiency and priorities in the first place? There's a trap in assuming the most reasonable version where conflicting instincts are involved. Not to mention survival of the family would've been in danger only if it followed "feed everyone" rule in the first place. There's enough of examples where "parents eat first" to doubt that.
As for the success - there's more to life/death than quantifiable goods that have to be divided. Disease, climate, accidents, conflicts, mistakes, birth itself, anything out of the ordinary - they all can produce an impressive death count on their own. Plus - even if we choose to believe those nomadic societies were trying to stick to 2+2 strategy, how could they even dream of accounting for infant mortality rate in any other way than... a "shotgun approach"? If decent number of kids in an entire group was the only "social security" that could be counted on, they had no luxury of "wait and see", certainly not with their lifespan. If anything, agricultural society seems to have at least lesser amount of variables to account for in comparison.