What are the technical reasons porn sites have such low PageRank? None go over PR6. A lack of links from trusted sites? Rampant link exchanges with low quality sites? Penalties for affiliate links? Or is there a general penalty on the industry?

Should "back to top" links have rel="nofollow"?

Underscores vs hyphens in URLS, does it make a difference? my-page vs. my_page?