I'm a single man in my 30s, and I don't want a relationship right now. I keep meeting women online who say they only want something casual. Then, on the first or second date, it becomes obvious that they want a relationship, not just fun and sex. What's with the bait and switch?

--Annoyed

Nothing like arriving for your reservation at a steak house only to be told, "We're out of a few things tonight -- everything made of cow. But good news! We've still got carrot kebabs, cauliflower schnitzel, and kelp stroganoff!"

Women who bait and switch like this -- revealing their relationshippy intentions between the appetizers and the end of date two -- are reflecting what evolutionary psychologists David Buss and David Schmitt call men's and women's conflicting "sexual strategies." These are best summed up as "happily ever after" for women versus "hookupily ever after" for men.

These differences in sexual strategy trace to differences in "obligatory parental investment." This refers to how a man can bolt after sex -- "Thanks, but I'll pass on doing the dad thing!" -- while a woman can get pregnant and stuck with a kid to drag around and feed.

Accordingly, Buss and Schmitt explain that women typically benefit most from a "long-term sexual strategy," vetting men to see whether they'd commit, meaning stick around to invest in any children that might come out of sex. Men, however, benefit most (that is, leave more descendants carrying their genes) from a "short-term sexual strategy" -- having casual sex with a variety of hot-erellas.

This doesn't mean that men never want to commit or that women never want to hook up. They do this when circumstances make it in their best interest. But because men and women coevolved, they are at least subconsciously aware of each other's intentions and shade the truth to put themselves in the most "marketable" light. So, men often act more interested in commitment than they actually are (in hopes of getting sex) and women often act less interested, in hopes of ensnaring Harry Hookup and turning him into Harry the Husband.

It probably makes sense to err on the side of assuming a woman will want commitment, whether she knows or articulates that or not. Opt for my "cheap, short, and local" advice for first and second dates: Meet for happy hour drinks or coffee for an hour or two, max. You still might get women who said they just want casual fun going gooey on you at the end of date two. At least you won't have shelled out for filet mignon and fine wine only to hear the no-strings-attached sex version of "First 100 callers get a free TV!" ... "Oh, sorry, sir...you're caller 101."

For pages and pages of "science-help" from me, buy my latest book, "Unf*ckology: A Field Guide to Living with Guts and Confidence." It lays out the PROCESS of transforming to live w/confidence.