According to Arthur Brooks in the Wall Street Journal:
liberals have a big baby problem: They’re not having enough of them, they haven’t for a long time, and their pool of potential new voters is suffering as a result.
According to the 2004 General Social Survey, if you picked 100 unrelated politically liberal adults at random, you would find that they had, between them, 147 children. If you picked 100 conservatives, you would find 208 kids. That’s a “fertility gap” of 41%.
Given that about 80% of people with an identifiable party preference grow up to vote the same way as their parents, this gap translates into lots more little Republicans than little Democrats to vote in future elections. Over the past 30 years this gap has not been below 20%–explaining, to a large extent, the current ineffectiveness of liberal youth voter campaigns today.
Alarmingly for the Democrats, the gap is widening at a bit more than half a percentage point per year, meaning that today’s problem is nothing compared to what the future will most likely hold.
However, the “Statistical Modeling, Causal Inference, and Social Science” blog has a take on the stats that provides some hope:
the trends that Arthur Brooks identifies are interesting, and I’d assume they’ll have some effect; at the same time, I’d be wary of using them to forecast too directly since the parties have the opportunity to change their positions while this is all happening.
[w]ay back when, people considered the demographic trends in the other direction, and expected that universal suffrage would lead to confiscatory taxation (the lower 60% of income could tax the upper 40% out of existence, and this would just continue because the poor have more kids than the rich), but it didn’t happen.
That’s the thing about the future: it’s, like, hard to predict and stuff.