Why do some parents prefer some children more than others? Is it true that parents tend to love or have a higher preference for some particular children of theirs?

If so, why?

Have any studies stated which of their children they love or prefer more?
There is quite a lot of research on parental favoritism. I'll summarize some of the findings:


girls are more often parental favorites (Harris &amp; Howard, 1985)
women more often report parental favoritism (Chalfant, 1994)
parents favor less aggressive children (which partially explains their preference for girls; Tucker et al., 2003)
the youngest child is more often the parental favorite (Harris &amp; Howard, 1985; Rhode et al., 2003; Tucker et al., 2003)
the middle child is least often the parental favorite (Harris &amp; Howard, 1985; Hertwig et al., 2002)
19% of 1st-born siblings, 30% of middle-born, and 26% of last-born claimed to be their father's favorite (Chalfant, 1994)
23% of 1st-born, 10% of middle-born, and 28% of last-born siblings claimed to be their mother's favorite (Chalfant, 1994)
favoritism was most often reported from a parent of the opposite sex (Chalfant, 1994)
only about 10% of children and 10% of parents felt or admitted that they had been favored or favored a child (Bedford, 1992)
more than 75% of the siblings felt themselves to be the favorite of at least one of their parents (Kiracofe &amp; Kiracofe, 1990)


As you can see, findings are somewhat contradictory.

Parental favoritism has consequences:


Self-esteem in the child correlates to perceived parental favoritism (Felson &amp; Zielinski, 1989; Zervas &amp; Sherman, 1994). Children, whose parents had treated all children equally, rated highest in self-esteem; favored children rated lower, nonfavored lowest (Zervas &amp; Sherman, 1994). Parents' supportive behavior affects the self-esteem of children but child self-esteem also affects how much support children report their parents give them; in addition, parents have a greater effect on the self-esteem of girls than of boys (Felson &amp; Zielinski, 1989).
Siblings express less warmth and greater hostility toward one another when parents show favoritism (McHale et al., 1995)
over time, being unfavored by parents produces behavior problems in children (Richmond, Stocker, &amp; Rienks, 2005)


Explanations for parental favoritism include:


Stress predicts favoritism. Favoritism is more common when parents experience marital problems and when children have serious health problems (Singer &amp; Weinstein, 2000).
Personality of the child predicts favoritism (see e.g. aggression above).


There is research on favoritism of children for parent also, as well as favoritism of adult children, both of which I did not report.

All of the above is limted to Western cultures. As we know, there are some cultures where parents go so far in their favoritism that they kill female children. The conclusion is that favoritism is not universal but culture specific.



Note: Most of the text above is copy/pasted directly from the quoted papers. I did not mark it as quotes as this would have greatly reduced readability.

Sources


Bedford, V. H. (1992). Memories of parental favoritism and the quality of parent-child ties in adulthood. Journal of Gerontology, 47(4), S149-S155.
Chalfant, D. (1994). Birth order, perceived parental favoritism, and feelings toward parents. Individual Psychology: Journal of Adlerian Theory, Research &amp; Practice.
Felson, R. B., &amp; Zielinski, M. A. (1989). Children's self-esteem and parental support. Journal of Marriage and the Family, 727-735.
Harris, I. D., &amp; Howard, K. I. (1985). Correlates of perceived parental favoritism. The Journal of genetic psychology, 146(1), 45-56.
Hertwig R., Davis J.N., &amp; Sulloway F.J. (2002). Parental investment: How an equity motive can produce inequality. Psychological Bulletin, 128, 728–745.
Kiracofe, N. M., &amp; Kiracofe, H. N. (1990). Child-perceived parental favoritism and birth order. Individual Psychology: Journal of Adlerian Theory, Research &amp; Practice.
McHale S.M., Crouter A.C., McGuire S.A., &amp; Updegraff K.A. (1995). Congruence between mothers' and fathers' differential treatment of siblings: Links with family relations and children's well-being. Child Development, 66, 116–128.
Richmond M.K., Stocker C.M., &amp; Rienks S.L. (2005). Longitudinal associations between sibling relationship quality, parental differential treatment, and children's adjustment. Journal of Family Psychology, 19, 550–559.
Rohde, P. A., Atzwanger, K., Butovskaya, M., Lampert, A., Mysterud, I., Sanchez-Andres, A., &amp; Sulloway, F. J. (2003). Perceived parental favoritism, closeness to kin, and the rebel of the family: The effects of birth order and sex. Evolution and Human Behavior, 24(4), 261-276.
Tucker C.J., McHale S.M., &amp; Crouter A.C. (2003). Dimensions of mothers' and fathers' differential treatment of siblings: Links with adolescents' sex-typed personal qualities. Family Relations, 52, 82–89.
Zervas, L. J., &amp; Sherman, M. F. (1994). The relationship between perceived parental favoritism and self-esteem. The Journal of genetic psychology, 155(1), 25-33.