Some populations practice having as many children as they can because their belief system calls for it. They believe they will get more brownie points from a supreme being for that.
So these groups will get more share of the basic income in the next generation and more in the next and so on and so on...
The basic income should be on a sustenance level. So even without any controls, having more children is not a way to get wealthy. It's basically like now; if some of them succeed it may pay off, but it's anti-social and usually doesn't work.
So these groups will get more share of the basic income in the next generation and more in the next and so on and so on...