Having children is a choice. There are no storks dropping babies into random homes.
True, people do make bad choices for a variety of reasons (teen pregnancy, for example), but nearly every parent, including teens, have some idea that children are a major responsibility that require a considerable amount of care, attention and financial resources from their parents.
This is not simply a matter of social/economic conditions or of genes. I remember being a teenager not all that long ago (I'm 32 now). The girls that got pregnant all knew better - I remember sitting through the same lectures in class that they did - but they made their choices.
So if it's not social conditions or genes, what is it?
I really think it boils down to ideas, which are accessible to anyone and everyone:
1) Life is for keeps. You only get one go at it, and that's it!
2) Your own life matters most, and it is possible to make something of it, if you try.
3) What's good for you does not come at the expense of other people.
4) How can you make your mark on the world if you're just starting out and all of your spare time is spent changing diapers?
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Your ideas are not determined by your social/economic conditions.^ If the ideas people held were simply a matter of economic conditions, no one would ever rise out of poverty. (I'd supply some names, but it's better if you just do a Google search for "poor people who became rich.")
^Nor are they determined by your genes - I won't delve into this because I'm assuming that this point is obvious.