Anytime I see talk of mass colonization as part of a solution to overpopulation, I direct people to Charles Stross' discussion about the infeasibility of doing so [1] [2]. The TL;DR is: until we find a breakthrough like cheap anti-gravity (like depicted in Interstellar), moving that much sheer mass of humanity is energetically (with the required specific impulse) not within our civilization's current capabilities.

[1] http://www.antipope.org/charlie/blog-static/2007/06/the_high...

[2] http://www.antipope.org/charlie/blog-static/2010/08/space-ca...

There is nothing reasonable about moving to Mars - a place where the average temperature is -55C and there is no breathable atmosphere. Imagine such a place on earth, and now imagine trying to sell a home there to someone.

We may, one day, colonize other planets. But we evolved specifically to exist on this one. Each new environment will likely take centuries, if not millenia, to properly colonize. The fact that it is theoretically possible for a comically tiny fraction of Earth's population to eke out a hardscrabble, miserable life on other planets should not impact policy decisions on terra firma.

Until you have hard data on how people born and raised in <40% Earth gravity turn out, moving to other planets is only a possibility, not something you can plan for.

Also, with new technologies like CRISPR, AI, and bionics (and beyond) will we soon dramatically alter fundamentally what it means to be human? Perhaps on a unit basis our future selves will be far more resource efficient.