Often, the first born is the recipient of the parents' favoritism, whether deserving or not. This is usually an accident of habit. The eldest receives more the exclusive attention of the parents in the critical early stages of life, which may result in slightly higher IQ, or a slightly bigger frame. There's also a cultural more that endures in families, lifting the first born to a status of, if nothing else, "the responsible one". Having been first born myself, I can identify with this moniker, though of course my mother still denies any favoritism among us growing up. (Her children know better.) By contrast, the frailest or most underachieving of the offspring should be the marker for the greatest disdain, and often they are. However, according to Kluger, a much greater percentage of parents possess a clear favorite, with the rest of their children occupying a roughly equal position in second place.
Gender plays a role within favored offspring as well, as the gender of the parent determines what characteristics they find most appealing. "Instead, what parents seem to value most in their opposite-sex children are the traits that, paradoxically, are associated with their own sex," writes Kluger, "the sensitive mom with the poetic son, the businessman dad with the M.B.A. daughter." These tendencies were outlined in a 2003 study of gender preferences by parents, but they show those primordial preferences in reproductively successful offspring to be present in modern parents.
Psychologically we are fallible people, and as parents we are not likely to actually possess a culturally imposed indifference between our children. Of course, it's our actions and our words that will convince our unfavored children that they are just as deserving of our love, or communicate a clear winner in the race for our attention.