How can it be that they can only have kids at ages 18-37?
That means they can only have kids twice in their lives...
By your own rules every COUPLE is sposed to have kids TOGETHER twice in their life...
Does that mean that every individual can have kids only with 2 others?

I may have confused myself there...all the above disregarding the rule of no interbreeding...We're taking that for granted...I'd really like to try and figure this out if you could gimme some info!!!

Dun take me wrong...all the above is subject to mistake...I'm no mathematician..