Excerpts from Polygamy and Population Growth by Mormonr.org:
Jacob 2 states, "For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people." ...
Did polygamy actually increase the population in Utah more than it would have otherwise?
Yes, probably. Studies have shown that societies where a man can have more than one wife tend to have many more children than other societies,[2] and Latter-day Saints had a lot of children relative to the rest of the U.S. at the time, particularly while polygamy was practiced.[3] ... Polygamy leads to fewer unmarried women,[7][8] which increases the number of children born in the polygamous society overall even if plural wives had fewer children than non-plural wives in that society.[9] ...
OUR TAKE [at Mormonr.org]
... The scriptures tell us that polygamy was to "raise up seed"—did that actually work?
There is some data from census records and other studies on how polygamy affected the Utah population. That data indicates that polygamy increased the population more than comparable non-polygamous populations at the time. It seems that though polygamous wives had fewer children, polygamous populations disproportionally increased because more women were married and could have children.
Understanding polygamy, and the reasons for it, is more complicated than simply stating that it was meant to increase the population. There are many more legitimate questions about polygamy, and it's okay to acknowledge the messiness and complexities of the topic. Commandments from God can have temporal justifications, but Latter-day Saints rely on faith as they make sense of something like polygamy.
My take on this is that plural marriage was practiced in order to raise up the seed of Ephraim among the first LDS members in the 1800s. So their was a selective birthing process where those from the "northern countries" (mostly from Britain and Scandinavia), that were declared the pure blood of Ephraim, were raising up their seed in particular. So that plural marriage selected for more of the Ephraimite women by giving them more opportunities to give birth to the seed of Ephraimite men at the time.
