(3 of 7)
That, as any parent knows, is a scenario trip-wired for fights--and that's what happened. The experimenters ranked the conflicts on a five-point scale, with one being a single cross word and five being a full-blown brawl. The next year, they went to the same children's schools to observe them at play and interview their teachers. Almost universally, the kids who practiced the best conflict-resolution skills at home carried those abilities into the classroom.
Certainly, there are other things that could account for what makes some kids battlers in school and others not. But the most powerful variables--parents and personality--were identified and their influence isolated during the course of the two-year-long observations. Socioeconomic status, an X factor that bedevils studies like this one, was controlled by selecting all the families from the same economic stratum. Distill those influences away and what is left is the interaction of the sibs. "Siblings have a socializing effect on one another," Shaw says. "When you tease out all the other variables, it's the play styles that make the difference. Unlike a relationship with friends, you're stuck with your sibs. You learn to negotiate things day to day."
It's that permanence, researchers believe, that makes siblings so valuable a rehearsal tool for later life. Adulthood, after all, is practically defined by peer relationships--the workplace, a marriage, the church building committee. As siblings, we may sulk and fume but by nighttime we still return to the same twin beds in the same shared room. Peace is made when one sib offers a toy or shares a thought or throws a pillow in a mock provocation that releases the lingering tension in a burst of roughhousing. Somewhere in there is the early training for the e-mail joke that breaks an office silence or the husband who signals that a fight is over by asking his wife what she thinks they should do about that fast-approaching vacation anyway. "Sibling relationships are where you learn all this," says developmental psychologist Susan McHale of Penn State University. "They are relationships between equals."
• How not being Mom's favorite can have its advantages
MULTICHILD HOUSEHOLDS CAN BE NOTHING short of palace courts, with alliances, feuds, grudges and loyalties, all changing day to day. Perhaps the touchiest problem in most such families is favoritism.
Parents feel a lot of guilt over the often evident if rarely admitted preference they harbor for one child over another--the sensitive mom who goes gooey over her son the poet, the hard-knocks dad who adores his tough-as-nails daughter. If favorites exist, however, it may be not the parents' fault, but evolution's.