i'm not tattooed or pierced so I don't why people do it, but I just suspect there are as many reasons as there are people. Rebellion against authority, aesthetics etc., many reasons. And some societal mores condemn for as many reasons.

I've heard some lame evolutionary theories before, but this may be the worst. Absolute rubbish in my opinion. I like tattoos. Always have. Therefore, I get tattoos. I prefer symmetry to randomness, so mine tend to be "balanced," [but I also like single sleeves, etc.] However, I seriously doubt this has anything to do with my ancestors creeping around in the primordial ooze.

Interesting study but there's no science in that third paragraph, just some unsubstantiated speculation. The general public often fails to grasp that it's much easier to collect data than it is to reach firm, scientifically valid conclusions based on that data, but actual scientists ought to know better.

What's clear to me is that having tattoos and piercings has become far more common and socially acceptable in recent years than it ever was throughout the previous years of my lifetime. The ready availability of colored inks is one factor, especially for girls. There definitely is an aspect of social approval involved, especially for younger people. More than that, I would have to say that in 2010, any girl in her late teens or early twenties who has zero tattoos or piercings (setting religion-based abstinence aside) is likely to be seen as a social outcast.

But when you try to figure out what it all means, things get murky very fast. It's possible there could be some important long-term social significance, but it's also possible it could simply be a fad. I doubt that there's any definitive scientific answer to the question right now. Time will tell...