Parental investment, in evolutionary biology and evolutionary psychology, is any parental expenditure (e.g. time, energy, resources) that benefits offspring. Parental investment may be performed by both males and females (called biparental care), females alone (exclusive maternal care) or males alone (exclusive paternal care). Care can be provided at any stage of the offspring's life, from prenatal (e.g. egg guarding and incubation in birds, and placental nourishment in mammals) to postnatal (e.g. food provisioning and protection of offspring).

Parental investment theory, a term coined by Robert Trivers in 1972, predicts that the sex that invests more in its offspring will be more selective when choosing a mate, and the less-invested sex will have intra-sexual competition for access to mates. This theory has been influential in explaining sex differences in sexual selection and mate preferences throughout the animal kingdom and in humans.

Sexual selection is an evolutionary concept that has been used to explain why in some species males and females behave differently in selecting mates. In 1930, Ronald Fisher wrote The Genetical Theory of Natural Selection, in which he introduced the modern concept of parental investment, introduced the sexy son hypothesis, and introduced Fisher's principle.

In 1948, Angus John Bateman published an influential study of fruit flies in which he concluded that because female gametes are more costly to produce than male gametes, the reproductive success of females was limited by the ability to produce ovum, and the reproductive success of males was limited by access to females. In 1972, Robert Trivers continued this line of thinking with his proposal of parental investment theory, which describes how parental investment affects sexual behavior. He concluded that whichever sex has higher parental investment will be more selective when choosing a mate, while the sex with lower investment will compete intra-sexually for mating opportunities. In 1974, Trivers extended parental investment theory to explain parent–offspring conflict, the conflict between the amount of investment that is optimal from the parent's perspective, versus from the offspring's perspective.

Parental investment theory is a branch of life history theory. The earliest consideration of parental investment is given by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection, wherein Fisher argued that parental expenditure on both sexes of offspring should be equal. Clutton-Brock expanded the concept of parental investment to include costs to any other component of parental fitness.^{[citation needed]}

Male dunnocks (Prunella modularis) tend to not discriminate between their own young and those of another male in polyandrous or polygynandrous systems. They increase their own reproductive success through feeding the offspring in relation to their own access to the female throughout the mating period, which is generally a good predictor of paternity. This indiscriminate parental care by males is also observed in redlip blennies (Ophioblennius atlanticus).

In some insects, male parental investment is given in the form of a nuptial gift. For instance, ornate moth (Utetheisa ornatrix) females receive a spermatophore containing nutrients, sperm and defensive toxins from the male during copulation. This gift, which can account for up to 10% of the male's body mass, constitutes the total parental investment the male provides.

In some species, such as humans and many birds, the offspring are altricial and unable to fend for themselves for an extended period of time after birth. In these species, males invest more in their offspring than do the male parents of precocial species, since reproductive success would otherwise suffer.