It's common to make inclusion into the tribe a difficult or expensive thing to do, so that you can more easily differentiate outsiders who may be a drain on your tribe's finite resources.

If you're curious about questions like this, Pinker's "How the Mind Works" is a very reasonable and readable take on evolutionary psychology.

"friction" in other words a barrier to entry to get filter those seeking the low hanging fruit.

Thanks for the book suggestion I've been looking for something along those lines..

Also see: fraternity hazing, boot camp, making the newest person at work your bitch and having him/her get the coffee and shine your shoes.