Parental investment (PI), in evolutionary biology and evolutionary psychology, is any parental expenditure (time, energy etc.) that benefits one offspring at a cost to parents' ability to invest in other components of fitness (Clutton-Brock 1991: 9; Trivers 1972). Components of fitness (Beatty 1992) include the wellbeing of existing offspring, parents' future reproduction, and inclusive fitness through aid to kin (Hamilton, 1964).
It is held that parental investment starts from the point when the male and female copulate and the egg is fertilized. The minimal obligatory parental investment for a human male is the effort required to copulate. On the other hand, the minimal obligatory parental investment for a human female is copulation, nine months of pregnancy and delivery. In that case the female investment outweighs the male investment. The difference of minimal obligatory investment between males and females suggests that the amount of investment and effort put into mating and parenting will also differ. In theory, a human male could impregnate any reproductive age woman who is fertile, leading to a large number of offspring from the male. In contrast, a human female can typically have only one offspring in nine months, limiting the amount of children she can have. This suggests that males should be more competitive between one another and females will be more ‘choosy’ because of the amount of investment, searching for the male with best fitness and good genes to pass onto her offspring (Trivers 1972).
Parental investment theory accounts for many of the differences between males and females: these were evolved in order to survive and reproduce. The importance can be seen in modern humans. Human males spend more time caring for their offspring than other male mammals (Bjorklund&Shackelford 1999). This higher parental investment is the result of extended childhood of human offspring. Prolonged human childhood is required in order to develop the brain. Optimally, children learn how to survive, as well as learning about the society and its vices and virtues. However, this requires parental investment in the form of parents ‘leading the way’- teaching and protecting children. Abandoned children may be left to die, though in some cases societies have developed various means of caring for them. Males do spend time caring for their children but to a much smaller degree than mothers. This translates into a general observation that females’ parental investment is much greater than that of males, both before and after childbirth.
Sexual Selection and Parental Investment 
Sexual selection is an important part of parental investment. Females will not only choose males with good fitness and genes, they will search for those with high status and resources and those who indicate the interest to invest in the offspring after it’s born. This is not only relevant in the Homo sapiens but also in the animal kingdom. For instance, the alpha lion will usually be the one who copulates with most if not all the female members of the group. Large amount of resources and territory may be attractive for females because it will secure a healthy environment for their offspring, indicating reproductive success. However, this also suggests that there will be a greater variance of successful mating in males than females.
In species where both sexes invest highly in parental care, mutual choosiness is expected to arise. An example of this is seen in Crested Auklets, where parents share equal responsibility in incubating their single egg and raising the chick. In Crested Auklets both sexes are ornamented.
Parental Investment and Parental Care 
Parental investment theory is a branch of life history theory. The earliest consideration of parental investment is given by Fisher (1930), Fisher's principle, wherein Fisher argued that parental expenditure on both sexes of offspring should be equal. Clutton-Brock (1991: 9) expanded the concept of PI to include costs to any other component of parental fitness.
Robert Trivers' theory of parental investment predicts that the sex making the largest investment in lactation, nurturing, and protecting offspring will be more discriminating in mating; and that the sex that invests less in offspring will compete for access to the higher-investing sex (see Bateman's principle). Sex differences in parental effort are important in determining the strength of sexual selection.
Reproduction is costly. Individuals are limited in the degree to which they can devote time and resources to producing and raising their young, and such expenditure may also be detrimental to their future condition, survival, and further reproductive output.
However, such expenditure is typically beneficial to the offspring, enhancing their condition, survival, and reproductive success. These differences may lead to parent-offspring conflict. Parental investment can be provided by the female (female uniparental care), the male (male uniparental care), or both (biparental care). Parents are naturally selected to maximise the difference between the benefits and the costs, and parental care will tend to exist when the benefits are substantially greater than the costs.
Parental care is found in a broad range of taxonomic groups, including both ectothermic (invertebrates, fish, amphibians and reptiles), and endothermic (birds and mammals) species. Care can be provided at any stage of the offspring's life: pre-natal care including behaviours such as egg guarding, preparation of nest, brood carrying, incubation, and placental nourishment in mammals; and post-natal care including food provisioning and protection of offspring.
A study found that male Dunnocks tend to not discriminate between their own young and those of another male in polyandrous or polygynandrous systems. However, 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.
In iteroparous species, where individuals may go through several reproductive bouts during their lifetime, a tradeoff may exist between investment in current offspring and future reproduction. Parents need to balance their offspring's demands against their own self-maintenance. This potential negative effect of parental care was explicitly formalised by Trivers (1972), who originally defined the term parental investment to mean any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.
The benefits of parental investment to the offspring are large and are associated with the effects on condition, growth, survival, and ultimately on reproductive success of the offspring. However, these benefits can come at the cost of parent's ability to reproduce in the future e.g., through increased risk of injury when defending offspring against predators, loss of mating opportunities whilst rearing offspring, and an increase in the time interval until the next reproduction.
Overall, parents are selected to maximise the difference between the benefits and the costs, and parental care will be likely to evolve when the benefits exceed the costs.
See also 
- Bateman's principle
- Cinderella effect
- Cost of raising a child
- Kin selection
- r/K selection theory
- Paternal care
- Amundsen, Trond (1). "Why are female birds ornamented". Trends in Ecology & Evolution 15 (4): 149–155. doi:10.1016/S0169-5347(99)01800-5.
- Bateman, A. J. (1948). "Intra-sexual selection in Drosophila". Heredity 2 (Pt. 3): 349–821. doi:10.1038/hdy.1948.21. PMID 18103134.
- Burke, T., Davies, N. B., Bruford, M. W., Hatchwell, B. J. (1989). Parental care and mating behavior of polyandrous dunnocks Prunella modularis related to paternity by DNA fingerprinting. Letters to Ecology.
- Beatty, John. 1992. "Fitness: theoretical contexts," in Key Words in Evolutionary Biology. Edited by EF Keller and EA Lloyd, pp. 115–9. Cambridge, MA: Havard U.Press.
- Clutton-Brock, T.H. 1991. The Evolution of Parental Care. Princeton, NJ: Princeton U. Press.
- Clutton-Brock, T.H. and C. Godfray. 1991. "Parental investment," in Behavioural Ecology: An Evolutionary Approach. Edited by J.R. Krebs and N.B. Davies, pp. 234–262. Boston: Blackwell.
- Hamilton, W.D. 1964. The genetical evolution of social behavior. Journal of Theoretical Biology 7:1-52.
- Trivers, R.L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man, 1871-1971 (pp. 136–179). Chicago, IL: Aldine. ISBN 0-435-62157-2
- Buss, D.M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures.
- Bjorklund, D. F., & Shackelford, T. K. (1999). Differences in parental investment contribute to important differences between men and women.
- Belsky, J., Steinberg, L., & Draper, P. (1991). Childhood experience, interpersonal develop- ment, and reproductive strategy: An evolutionary theory of socialization.
- Kevin Woodward and Miriam H. Richards (2004) The parental investment model and minimum mate choice criteria in humans
Further reading 
- Geary, D. C. (2005). Evolution of paternal investment. In D. M. Buss (Ed.), The handbook of evolutionary psychology (pp. 483–505). Hoboken, NJ: John Wiley & Sons. Full text