Well, if militant vegetarians/vegans could have their way, it would no doubt have a huge effect on population. If malnourished mothers carry children to term, those children would have permanent cognitive deficits, leading to a decrease in the average population IQ. Many of the children will not survive to term. I do not think it would be possible to force vegetarianism on a population, though. People would use whatever means possible to get protein—there would be black-market meat.
True.