SCRUBBED UP AND SLICKED down for a holiday photo, many brothers and sisters look alike. But you don't have to be a parent to know that looks are deceiving; when it comes to personalities, passions and talents, siblings can be as different as a mixed pod of sweet and black-eyed peas.
Why is that? You might guess inborn predilections, and you would be partly right: Each brother and sister gets a different mix of genes from Mom and Dad, so it makes sense that whatever power nature wields over personality traits such as optimism or anxiousness shapes each sibling a little differently.
But surely the effects of nurture -- the neighborhood, the schools, the quality of Mom and Dad's marriage and child-rearing style -- all work to make siblings similar, right?
Wrong. Very, very wrong, according the Pennsylvania State University child psychologists Robert Plomin and Judy Dunn, who have been busy the last few years toppling conventional ideas about the seeds of personality.
It's not all genetic, they say, and it's not all environmental. Sounds pretty tame so far, but their suggestion isn't the pat compromise it seems.
Let's get personal: According to Plomin and Dunn, your siblings are no more like you than if they'd switched homes their first day of life and grown up in, say, your boss' or dentist's family.
"It's startling at first," Plomin admits. "But all the evidence points to the same conclusion: What we've thought of all along as 'shared family environment' doesn't exist."
That's because we each carry around between our ears our own little customized version of our environment. From our first days of life, and perhaps even before, we perceive everything that xTC happens to us through a unique filter, every skewed event changing us in a way that affects how we'll experience the next event.
In fact, says Plomin, it looks as though growing up in the same familial world actually works to make siblings different.
Take Orioles Cal and Billy Ripken. Of the two brothers, Cal is more private, even-tempered, not the type to argue with an umpire. In fact, when he was thrown out of a game last year, the umpire who gave him the heave remarked that it was like "throwing God out of Sunday school." Little brother Bill, on the other hand, is more outgoing, demonstrative of his emotions, a practical joker. The younger Rip was involved in a scandal of sorts a few years back, when early editions of his baseball card included an obscenity written on the end of his bat. The only intrigue surrounding Cal concerns whether he drinks whole or skim milk.
Plomin is an old soldier in the nature/nurture wars. Since 1975, he has studied twins and adopted children, searching for clues that can tell us which aspects of personality are inherited and which are learned.
The quirkiest findings from such studies have made flashy headlines. There are tales of identical twins -- genetic replicas -- who were separated at birth and reared in different families without any contact with each other, only to grow up to be remarkably similar adults. One pair became fire department captains, for example. Another twosome shared the same goofy appreciation for practical jokes, while a third pair favored gaudy jewelry.
Those are just the bells and whistles of a more generalized genetic influence that most psychologists now agree seems at least as crucial as family environment in shaping human personality.
But Plomin and Dunn are more interested in the findings' flip side, deduced from children who had family environment in common but not heredity. When the researchers studied pairs of adopted siblings they were surprised to discover that on standard personality tests, the siblings scored no more similarly than would a couple of strangers.
There's no single explanation for the finding. Birth order, for example, doesn't account for the striking differences between siblings, Dunn says. The cherished notion that eldest children are more independent, responsible or creative simply because they came first is a particularly hardy myth, but studies looking for such effects are not consistent in their results.
Even if parents are universally less experienced with their first child than with their third or fifth, that difference gets filtered through their personalities and each child's to a very different end. The same 10 p.m. curfew that prompts rebellion in one budding teen may be welcomed by another.
If there is one factor that does show consistent importance in shaping differences among siblings, it is every child's fierce tendency to make comparisons in how he or she is treated. Dunn found in studies of children as young as 14 months old that long before they are good at conversation, children are acutely aware of the minute-by-minute differences in parental attention and affection doled out to their brothers and sisters -- as evidenced by their skill in yanking back the spotlight.
In fact, even parents who try to be evenhanded are foiled by their own consistency because they can't control the way their kids will perceive their efforts.
"When we followed the same families over a number of years, we found that most parents are remarkably consistent in the way they treat successive children at any particular age," Plomin says. " A mother may be very affectionate with kids when they are a year old, for example, but emphasize independence with 3-year-olds. It looks as though the 3-year-old pays more attention to her brother's being smothered with kisses than to the memory of being cuddled herself at that age.
Dunn, who observes children for a living, thinks parents should take away a different, more comforting message. "I'd say, 'Relax,'" she says. "I'm constantly struck by the power of children's personalities almost irrespective of what their parents