So, what's the catch about birthday rewards?

It was my birthday at the end of June. Don't feel bad if you forgot to send me a gift. My favorite restaurants and retailers filled the void with offers for free meals, generous coupons -- even a free bottle of sweet-smelling bath gel. All I did was sign up for several in-store or online loyalty programs, which asked for my birthdate. I found them through word-of-mouth and by scouring the Web. About two weeks before my birthday, e-mail coupons for freebies stuffed my mailbox. You might want to get a separate e-mail account if you're bothered by a cluttered in-box. But so many coupons and other consumer goodies arrive electronically these days that coupon-clippers will find it worthwhile.

I ignored some offers, such as the "free" items requiring me to purchase something in return for my gift.

But other offers were too good to pass up. I enjoyed a free entree at both Noodles & Co. and Red Robin with no strings attached. I stopped by Caribou Coffee for a huge frozen drink and had an ice cream cone on the house at Cold Stone Creamery.

Fortunately, I didn't have to eat it all in one day. Reading the fine print revealed that most places gave me 14 to 30 days to redeem my coupon.

The offers weren't all for food either. I received a free bottle of birthday-cake-scented bath gel at Sephora, although it was about the size you'd see in a hotel bathroom. I also used a $10 coupon at shoe store DSW and a $5 coupon at Famous Footwear. I bought socks at both stores and shelled out less than $2 to complete the transaction.

So what's in it for the retailers? Simple. The offers get you into stores with birthday money jingling in your pocket. "It all comes down to getting you to spend money," said Jay Siff, whose Pennsylvania company Moving Targets runs a birthday direct-mail service for restaurants and auto shops. A study by the Direct Marketing Association showed that free gift offers are the most widely used and effective marketing tool. Birthday offers are especially appealing because consumers are in a celebratory mood. Even skeptics are quicker to accept a birthday freebie from a company than a freebie sent for no reason.

'Predictably irrational'

Because most people will abide by the law of reciprocity, that "when someone does something for us, we want to do something back," said Siff.

But it's a retail chain, I countered, not a person. Siff still thinks the principle works. Maybe I didn't buy a drink to wash down my free noodles, but if I have to choose a restaurant down the road, Siff bets I'll be "somewhat inclined to deal with the [restauarant] that has done something nice." We'll see.

Free, of course, has a price. Just ask behavioral economist Dan Ariely, who said he picked out the wrong car because he was offered free oil changes for three years. I love Ariely's research style. The author of "Predictably Irrational: The Hidden Forces that Shape Our Decisions," notices something irrational that he's doing and studies whether others tend to make the same mistakes. His studies found that the word "free" blinds us.

"When we get something for free, we don't think it involves any negatives," he explains. We also fail to consider opportunity cost. For example, Ariely asked me how much time I spent collecting my birthday "gifts." Did I drive all over town burning gas?

I tried to bundle my trips to save time and gas. But I admit that when vying for a parking spot at a crowded mall in order to score my free bottle of bath gel, I began to question my sanity. Then again, I felt a teensy thrill from my stash of goodies and my tummy full of free food. That must be worth something, right?