Since procreation is basically the biologically narcissistic act of replicating ourselves through succeeding generations, parents choose the biggest, healthiest offspring, as it is more likely to carry healthy genes into the next generation.
A crested-penguin mother will kick the smaller of her two eggs out of the nest. A black-eagle mother will watch idly while her bigger chick rips her smaller one to ribbons. The function of the second chick is insurance; if the first chick is healthy, the policy is cancelled.
Firstborns are often the family's favourite: the rule of sunk costs. The more effort you've made developing a product, the more committed you are to seeing it come to fruition.
The most likely candidate for the mother's favourite is the firstborn son, for the father, the lastborn daughter.
Favouritism can fluctuate, depending on family domains. An active child may be intolerable at home, but a darling on the field.
Kids who feel less loved are more likely to develop anxiety, low self-esteem and depression.
Favouritism conflicts fade as children get older.
Being the favoured may boost self-esteem and confidence but studies show it can also leave kids with a sense of arrogance and entitlement. Unfavoured children may grow up wondering of they're somehow unworthy of the love the parents lavish on the golden child. But they do better at forging relationships outside the family.