There's a new development in the ever-popular field of adultery today.  Claiming "it's not my fault, I'm a sex addict" is straight up 2009.  The hot new excuse for 2010 is . . . "it wasn't my fault, BIOLOGY made me do it."

According to Christopher Ryan, who wrote the new book "Sex at Dawn:  The Prehistoric Origins of Modern Sexuality", one of the biggest reasons a man might cheat is an uncontrollable surge of testosterone.

By the time a man's in his 20s, his testosterone level starts to go down.  As it goes down, so does a guy's passion and appetite for life. 

(It's why men are so willing to get married and suddenly be cool with trips to light fixture stores.)

But when a man has just a brief conversation with an attractive woman, his testosterone levels spike WAY up.

That spike can confuse the brain . . . and make a guy think he's just fallen in love with someone new.  And that can make the guy do something he'd regret later.