ead to fewer babies being born which would threaten a small group.
Why would fewer successful pregnancies be risky for survival? In the "wild," humans would be subject to being hunted, accidents, diseases, etc. To survive and prosper, you need to have lots of offspring to make up for this high rate of attrition. You'd really want to grow your little group as quickly as possible.
Conclusions. As you can see, this is a tough number to figure out. It all depends on the founders' genes. Too many "bad" genes with too few founders and you end up with a sickly group that probably won't survive. If you increase the number of people or decrease the number of "bad" genes, the group's chances go way up.
So, given the average number of bad genes in people, 160 is about the right number to give a good chance for survival. We'll have to build a pretty good-sized space ship to colonize another planet!
(Or maybe 70...a recent study at http://www.livescience.com/history/050525_america_settlers.html suggests that North America may have been settled by only 70 people!)