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Parental favouritism is the differential treatment of offspring or children by their parents. In evolutionary biology, it may be measured in terms of parental investment. This complex behaviour is seen in a variety of animal species, including Homo sapiens; factors which may influence parental favouritism include birth order, sex, and relatedness.

Primogeniture and ultimogeniture
Primogeniture refers to the practice of conferring inheritance onto the first-born offspring, to the exclusion of younger siblings. Charles Darwin is noted to have considered this practice a Darwinian puzzle, due to the seeming contradiction between parental favouritism based on overall fitness, and that using age as the sole variable. However, a possible explanation for the prevalence of this phenomenon in European inheritance laws may be found by considering the limits of parental investment.