The Rat Of The Litter

For two months, I've been dating an awesome guy. He does sweet things like leaving cute notes on my windshield, but I worry about how he looks up to his older brother, who isn't the greatest person. What's most worrisome is how his brother treats women like garbage, saying anything to get them into bed and then ditching them or cheating. I haven't known my apparently awesome boyfriend long, so part of me worries about whether any part of his brother has rubbed off on him or will. How much of a "family resemblance" is there between brothers?

--Having Cautious Fun

Younger brothers do tend to look up to older brothers, and frankly, this is hard to avoid if one's older brother is always dangling out some married woman's second-floor window. But behavioral science research finds that personality isn't transferred from one person to another like cat hair from a couch to black pants. "Personality similarity between relatives seems to come mostly from their shared genes," writes behavioral geneticist and twins researcher Nancy Segal in "Born Together--Reared Apart." About your boyfriend and his brother, Segal told me, "If they were identical twins, I would worry!" Identical twins share 100 percent of their genes, she explained. But "siblings share 50 percent of their genes, on average" and "can be very different." And even with those genes they share, biology isn't destiny. The same gene that vaults into action in one brother (sending chemical signals to the brain that influence personality) might spend a lifetime napping in the other. Gene expression -- whether certain genes get switched on -- is triggered by environment (which includes diet, chemical exposure, and a person's experiences). And although these brothers grew up in the same family, the same environment's effect on different siblings can be different because they experience it at different ages, with a different combination of genes, and with different peer and other influences. So, for example, four brothers can have the same physically abusive grifter father but only one of them -- executed murderer Gary Gilmore -- ends up a cold-blooded killer. And then there's Bill Clinton and his half brother Roger -- one of whom was the leader of the free world and the other, a leader in finding the free beer. Chances are your boyfriend looks up to his brother for historical reasons -- for building him forts out of couch cushions and making some bully wear girls underwear on his head -- and he doesn't want to mess up his misty view with new information, like how his brother collects girls' tears in little labeled glass vials. You, however, are on the right track -- "having cautious fun" instead of deciding your boyfriend's the cheese and closing your eyes to any information contradicting that. But while your boyfriend's brother is a user of people, which points to a lack of empathy, your boyfriend's behavior (just per the notes he leaves on your car) suggests he takes pleasure in delighting you, which suggests he truly cares about you. If only his brother would show similar thoughtfulness and start leaving his own cute notes on girls' cars -- perhaps something along the lines of "Roses are red, violets are blue; I just got a shot at the free clinic, and so should you."