I have a couple of theological and historical/statistical questions about the reasons for polygamy.
People who know more than I do have told me that widespread polygamy was not a response to an imbalance between the numbers of men and women. In Deseret the sexes were in roughly equal ratio.
They also say that polygamy was not an attempt to increase the birth rate. Deseret polygamy, like polygamy generally, seems to have lowered the birth rate at least a little. Neither the birth rate nor the sex imbalance are reasons that work.
Jacob quotes a divine reason in his sermon on concubines and whoredoms: “For if I will . . . raise up seed unto me, I will command my people.” Notice that this isn’t just the discredited concern about birth rates. The Lord says he wants not just to “raise up seed,” but to “raise up seed unto me.”
We cannot be sure that this is the reason for polygamy in Deseret times. Jacob was explaining why Abraham and Isaac practiced polygamy, not why Brigham Young and Brother Joe Blow did. And I’m pretty sure that the reasons Brigham Young and Brother Joe Blow gave for polygamy weren’t that it was a temporary thing necessary to raise up seed (though I could be wrong). On the other hand, D&C 130 does tie polygamy to Abraham, whose polygamy the verse in Jacob is explaining. Its probably worth trying to see if “raising up seed unto me” works as an explanation for polygamy in Utah. If we can.
If “seed being raised up unto me” means a greater number of people who are exalted, we have no way of measuring, unless we assume that there’s some here-and-now statistic that’s correlated with exaltation (children being raised in the church, e.g.) or if we adopt some Jan Shipps -style, double theological rimshot where polygamy makes the Church persecuted which in turn keeps it from dissolving which, long term, results in more members and more exaltation.
On the other hand, “seed being raised up unto me” could just mean (1) children being raised in the church, or (2) children who are in the covenant because both of their parents are faithful. If the modern data showing that women who marry gentiles or inactive saints are less likely to raise their kids Mormon applies to Deseret, then the two would tend to overlap considerably, though the Deseret practice of sealing outside family units might complicate this a little. In any case, either should be measurable in theory. At least in theory we should be able to show that while the birth rates over all may have been lower, the number of children raised in the church or with both parents active was higher than it otherwise would have been.
This leads us to an even greater puzzle. Even if we do establish that polygamy effectively “raised up seed unto me,” the reason why it ended in Deseret in particular is still no mystery. The US government had a gun to the Church’s head. That doesn’t explain why we don’t practice polygamy now, when the gun is largely removed. It also doesn’t explain why Jacob (or the Lord speaking through Jacob) would view polygamy as the exception rather than the rule. We would either have to think that there are other considerations that generally override the need for seed (Jacob suggests that the tender feelings of women are one such consideration), or that polygamy only “raises up seed unto me” under specific circumstances that don’t generally hold. A third explanation–that “raising up seed unto me” was only the purpose of Abraham’s polygamy, not polygamy generally–would work for the Church’s unwillingness to return to polygamy now but not the Book of Mormon refusal to do so.