Understand his mating mind-set: the key to knowing what makes a guy tick is figuring out the stage of life he's in. Cosmo interprets where your man's head (and heart) are so you can better date and relate to him.

Part of getting a handle on the dude you're seeing--and whether a long-lasting relationship is in the cards--depends on where he is in life. We used to think a man's age dictated his dateability, but it's actually his life stage that matters most. You see, there are four distinct phases of emotional development a guy can go through that have nada to do with his birthday. "Milestone activities can be as important as biology in determining a man's psychological state--his experiences shape him," says Michael Gurian, author of What Could He Be Thinking? and founder of the Gurian Institute, a research center studying file differences between the male and female brains.

And the most influential experiences tend to center around success. "Men define themselves based on their careers and financial status," explains Manhattan psychologist and relationship expert Mari Terzaghi, PhD.

So you can comprehend where your guy is coming from and where your relationship might be going, we got the lowdown on the main boyfriend phases. Find out where your man fits in.

IS HE ...

* Adventurous and into trying new things?

* Rarely, if ever, stressed out?

* Spontaneous--never makes plans in advance?