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Compassion — a feature not unique to humans but seen a lot more commonly in our species — was clearly at work here. But so were other practices we share with nonhuman species. In her book Mother Nature: A History of Mothers, Infants and Natural Selection, anthropologist Sarah Blaffer Hrdy of the University of California at Davis cites work conducted with coots, a species of black-and-white waterbird common in Europe and the Americas. Unlike other birds, coots don't pour most of their parenting efforts into their strongest chicks but rather spread the care around in the hope of maximizing the raw number of offspring that survive. This can mean not just remembering to treat the weakest of your offspring equally but favoring them, since they're the ones that need the help.
In case a mother forgets which chick is the youngest (coots do all look remarkably alike), nature provides an unmistakable cue in the form of a bit of fancy red plumage on babies' heads. Baby birds don't have the colorful tuft for long, but for the period they do, they are irresistible, and mothers with a nest of babies will steer extra food to the reddest head in the bunch. It is surely a coincidence that Bruce, the baby in our nest, was a redhead, but that doesn't mean his coloration didn't work its charms on all of us. The chapter in Hrdy's book that deals with the coot is called "Why Be Adorable?" The behavior of human and animal families answers that question neatly.
Across the Genders
If physical appearance can be such an important driver of favoritism, gender, which goes beyond mere looks and into the very essence of the child, should be even more powerful. And it is. The oft seen pattern of parents with cross-gender preferences in their kids — the dad who's helpless in the face of his daughter's charms or the mom who adores her prince of an eldest son — is one good example. Such favoritism patterns hardly exist in every family, but they're more common than we think, as Salmon discovered in a 2003 study that was published in the journal Human Nature. "I asked subjects to list which child in the family was their mother and father's favorite," she says. "Overall, the most likely candidate for the mother's favorite was the firstborn son, and for the father, it was the last-born daughter. "
Studies that have dug deeper into this preference have found that it's not just the frilliness of a little girl that appeals to Dad or the uncomplicated love that can come from a boy that delights Mom. And while Freudians would raise the oedipal specter, modern studies have marginalized that factor. Instead, what parents seem to value most in their opposite-sex children are the traits that, paradoxically, are associated with their own sex — the sensitive mom with the poetic son, the businessman dad with the M.B.A. daughter. Reproductive narcissism, again, may play a role there. It's not always easy for a father to see himself replicated in a daughter or a mother to see herself in a son. But if the kids can't look like you, they can at least act like you. Sometimes, children may come by these traits innately; sometimes it can be tactical, a way to court a little extra love. In this sense, kids are like tree leaves, sorting themselves out so that they grow in a shaft of light not blocked by the leaf above.
"Siblings are devilishly clever," says author and family expert Frank Sulloway of the University of California, Berkeley, "much smarter than psychologists. They are constantly trying to fine-tune their niche to squeeze the maximum benefits out of their parents."
Gender may be especially powerful in determining favoritism in three-child families. As a rule, first- and last-born children have a better shot at being at least one parent's favorite than middle kids do. In all-boy or all-girl families this is especially so, since the middle child stands out neither by birth order nor by sex. That's the case too in families in which the gender sequence is, say, boy-boy-girl or boy-girl-girl, since the middle child is still not unique. Shifting the sequence, however — to boy-girl-boy or girl-boy-girl — may change everything. In these cases, the uniqueness of gender can trump everything else.
"If you have a child who is different for any reason, especially being the only girl or only boy," says Salmon, "that child is going to get extra attention and investment."