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, and is thus a form of sexual selection. Components of fitness include the wellbeing of existing offspring, parents' future sexual reproduction, and inclusive fitness through aid to kin. Parental investment may be performed by both the male and female (biparental care), the mother alone (exclusive maternal care) or the father alone (exclusive paternal care).
Initially introduced in 1930 by the English biologist and statistician Ronald Fisher, 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.
An alternative to Fisher's theory is Trivers' parental investment theory (PIT) proposed in 1972, in which he focuses on how parental investment in humans affects their sexual behaviour. This theory has been influential in explaining sex differences in phenomena such as sexual selection and mate preference. The main premise of this theory is the differential investment of males and females, particularly how in most species females invest more due to the mating effort required. Another aspect of the theory is mate preferences of each sex, for example female choosiness and male promiscuity.
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 PI to include costs to any other component of parental fitness.
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 maximize the difference between the benefits and the costs, and parental care will tend to exist when the benefits are substantially greater than the costs.
Penguins are a prime example of a species that drastically sacrifices their own health and well-being in exchange for the survival of their offspring. This behavior, one that does not necessarily benefit the individual, but the genetic code from which the individual arises, can be seen in the King Penguin. Although some animals do exhibit altruistic behaviors towards individuals that are not of direct relation, many of these behaviors appear mostly in parent-offspring relationships. While breeding, males remain in a fasting-period at the breeding site for five weeks, waiting for the female to return for her own incubation shift. However, during this time period, males may decide to abandon their egg if the female is delayed in her return to the breeding grounds. This is an interesting case, as it shows that these penguins initially show a trade-off of their own health, in hopes of increasing the survivorship of their egg, but there comes a point where the male penguin's costs become too high in comparison to the gain of a successful breeding season. In a study, Olof Olsson investigated the correlation between how many experiences in breeding an individual has and the duration an individual will wait until abandoning his egg. He proposed that the more experienced the individual, the better that individual will be at replenishing his exhausted body reserves, allowing him to remain at the egg for a longer period of time. The males' sacrifice of their body weight and possible survivorship, in order to increase their offspring's chance of survival is a trade-off between current reproductive success and the parents' future survival. This trade-off makes sense with other examples of kin-based altruism and is a clear example of the use of altruism in an attempt to increase overall fitness of an individual's genetic material at the expense of the individual's future survival.
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. This indiscriminative parental care by males is also observed in redlip blennies.
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 formalized by Trivers in 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. For example, in the cichlid fish Tropheus moorii, a female has very high parental investment in her young because she mouthbroods the young and while mouthbrooding, all nourishment she takes in goes to feed the young and she effectively starves herself. In doing this, her young are larger, heavier, and faster than they would have been without it. These benefits are very advantageous since it lowers their risk of being eaten by predators and size is usually the determining factor in conflicts over resources. However, such 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.
A special case of parental investment is when young do need nourishment and protection, but the genetic parents do not actually contribute in the effort to raise their own offspring. For example, in Bombus terrestris, oftentimes sterile female workers will not reproduce on their own, but will raise their mother's brood instead. This is common in social Hymenoptera due to haplodiploidy, whereby males are haploid and females are diploid. This ensures that sisters are more related to each other than they ever would be to their own offspring, incentivizing them to help raise their mother's young over their own.
Overall, parents are selected to maximize the difference between the benefits and the costs, and parental care will be likely to evolve when the benefits exceed the costs.
Extensive parental investment is known in a wide range of small-brained animals, including some species of fish, amphibians and insects. Therefore, the bigger brains in mammals compared to most other animals cannot be due to a minimum parental care brain size. The fact that big brains consume more nutrients than smaller brains makes this especially relevant for evolutionary theories of the brain.
Parental care only requires behaviors that effectively enhances offspring's chances of survival, it does not require underlying mechanisms to be potentially at a continuum with generalizable empathy appliceable to adults, nor even other situations involving young than the specific reactions, nor does it require the offspring to be altruistic back in any way. Total ungeneralizability to adults has the evolvability advantage of not making parentally caring individuals vulnerable to being exploited by other adults.
In many species, sexual selection is closely linked to parental investment. In theory, a male from such a species can produce a large number of offspring over the course of his life by minimizing parental investment in favor of investing his time instead impregnating any reproductive-age female who is fertile. In contrast, a female from said species typically can have a much smaller number of offspring during her reproductive life, partly due to an obligatory non-nil parental investment (i.e., gestation and delivery). This suggests that females will be more selective ("choosy") of mates than males will be, choosing males with good fitness (e.g., genes, high status, resources, etc.), so as to help offset any lack of direct parental investment from the male, and therefore increase reproductive success. 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).
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.
The importance of parental investment can be seen especially in species in which the offspring are altricial (i.e., unable to fend for themselves from earliest ages). In many bird species and in modern humans, this leads to males spending more time caring for their offspring than do the male parents of precocial species, since reproductive success would otherwise suffer. The higher parental investment in humans is the result of an extended childhood required in order to develop the unusually large human brain. During this time, parental investment comes in the form of feeding, teaching, and protection, for example.
However, if human parental investment starts from the point when the sperm fertilizes the egg, then the minimal obligatory parental investment for the male is zero effort, while the minimal obligatory investment for the female is nine months of pregnancy, followed by delivery. This difference of minimal obligatory investment between males and females suggests that the amount of investment and effort put into mating and parenting will differ.
In some insects male parental investment is given in the form of a nuptial gift. For instance, ornate moth 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.
Parental investment as defined by Trivers in 1972 is the investment in offspring by the parent that increases the offspring's chances of surviving and hence reproductive success at the expense of the parent's ability to invest in other offspring. A large parental investment largely decreases the parents' chances of investing in other offspring. Parental investment can be split into two main categories: mating investment and rearing investment. Mating investment consist of the sexual act and the sex cells invested. The rearing investment is the time and energy expended to raise the offspring after conception. Women's parental investment in both mating and rearing efforts greatly surpasses that of the male. In terms of sex cells (egg and sperms cells), the female's investment is a lot larger, while males produce thousands of sperm cells which are supplied at a rate of twelve million per hour. Women have a fixed supply of around 400 ova. Also, the acts of fertilization and gestation occur in the women, which compared to the male's investment of just one cell outweighs it. Furthermore, each intercourse could result in a nine-month commitment such as gestation (the act of breastfeeding) for the woman. From Trivers' theory of parental investment several implications follow. The first is that women are often but not always the more investing sex. The fact that they are the more investing sex has meant that evolution has favoured females who are more selective of their mates to ensure that intercourse would not result in unnecessary costs. The third implication is that because women invest more and are essential for the reproductive success of their offspring they are a valuable resource for men; as a result, males often compete for sexual access to them.
Due to the heavy investment that women engage in after conception, evolution has favoured women who are more selective of their partners. Therefore, generally speaking, women are more discriminating about their partners. Their mate preferences usually include males which are of good financial status, and could therefore invest more resources; who are more committed, and would therefore dedicate more time and energy; who are more athletic, as they would be better at protecting and defending the female and her offspring; and who are healthier, again with the prospect in mind that they would ensure healthy offspring.
Women's preferences for mates, however, vary depending on their context. There are two contexts which have been identified to affect preferences. The first is women's personal resources, also referred to as the "structural powerlessness hypothesis". According to this hypothesis women strive to find mates which have access to resources mainly because they themselves are excluded from them and they are usually largely controlled by men. Therefore, women see finding a mate with a lot of resources as a channel to gain access to these resources. However, this hypothesis has been disproved by studies such as the one done by Buss  which found that successful women place even greater importance on financial and societal status as well as on professional degrees.
Another context which is known to alter female's preferences for mates is temporal boundaries. Studies by Buss and Schmitt  have found that women put great emphasis on career-orientation, ambition and devotion only when considering a long-term partner. When marriage is not involved often females would put greater emphasis on looks. This could be explained through parental investment theory because when women expect to have offspring from a particular mate they employ a more demanding criteria as they know it will affect the reproductive success of their offspring, however when looking at short-term mating which would not involve long-term commitments, females are less selective. Of course, this alternation is only possible for humans with birth control technology which was not available in the past.
For many species the only type of male investment received is that of sex cells. In those terms, the female investment greatly exceeds that of male investment as previously mentioned. However, there are other ways in which males invest in their offspring. For example, the male can find food as in the example of balloon flies. He may find a safe environment for the female to feed or lay her eggs as exemplified in many birds. He may also protect the young and provide them with opportunities to learn as in many young as in wolves. Overall, the main role that males overtake is that of protection of the female and their young. That often can decrease the discrepancy of investment caused by the initial investment of sex cells. And indeed there are some species such as the Mormon cricket, pipefish seahorse and Panamanian poison arrow frog males invest more. What is interesting is that among the species where the male invests more, the male is also the pickier sex placing higher demands on their selected female. For example, the female that they often choose usually contain 60% more eggs than rejected females. This links Parental Investment Theory (PIT) with sexual selection: where parental investment is bigger for a male than a female, it's usually the female who competes for a mate, as shown by Phalaropidae and polyandrous bird species. In these species females are usually more aggressive, brightly coloured, and larger than males, suggesting the more investing sex has more choice while selecting a mate compared to the sex engaged in intra-sexual selection.
The second prediction that follows from Trivers' theory is that the fact that women invest more heavily in offspring makes them a valuable resource for males as it ensures the survival of their offspring which is the driving force of natural selection. Therefore, the sex that invests less in offspring with compete among themselves to breed with the more heavily investing sex. In other words, males will compete for females. It has been argued that jealousy has developed to avert the risk of potential loss of parental investment in offspring. If a male redirects his resources to another female it is a costly loss of time, energy and resources for her offspring. However, the risks for males are higher because although women invest more in their offspring, they have bigger maternity certainty because they themselves have carried out the child. However, males can never have 100% paternal certainty and therefore risk investing resources and time in offspring that is genetically unrelated. Evolutionary psychology views jealousy as an adaptive response to this problem.
Very often parent-offspring conflict can find its roots in parental investment. Because parents are equally related to all offspring they try to distribute their investment equally and proportionally. However, because the offspring is more related to themselves than they are to their siblings, they often require a lot more care than the parent is able to provide. According to Trivers, this discrepancy in investment optima occurs because the parent is selected to invest in the offspring up until the point in which the cost of investing more in the current offspring, and hence leading to less future reproductive success, is larger than the benefit of increased survival of the current offspring. Conflict arises if the parent continues to invest in the offspring even to the critical point where the cost begins to outweigh the benefits.
From an evolutionary perspective, the sexual strategy and parental investment of male and female differ considerably and females usually invest more in parental care due to a time-consuming period of pregnancy and childbirth, while male are not restricted by the sperm production but by access to more sexual partners. There is a trade-off between mating effort and parenting effort, and males are generally more willing to have more sexual partners compared to females, as reproduction of more offspring instead of using the time and resource to invest in existing offspring would be a trade-off. Therefore, the amount and duration of investment necessary to ensure the survival of offspring was unequal between the two sexes, with females investing more.
On the other hand, Trivers has proposed that females prefer to mate with males who display both an ability and willingness to invest resources for the survival of the female and her offspring, thus, the costs of female parental care and investment could be balanced by benefits in terms of more paternal investment in resources for offspring, This would mean that females who expect paternal investment in resources such as territory will be more conservative and careful towards having sexual intercourse with males, regardless of how attractive the potential male is.
Sociosexual orientation refers to individual differences in willingness to engage in sexual relations without love or intimacy. Unrestricted sexuality refers to a kind of sexual strategy people adopt when they engage in more than one concurrent sexual relationship and have had sex with multiple partners, usually without commitment such as one-night-stands. Individuals with a restricted sexuality would often have closeness and commitment before engaging in a sexual relationship. Using socio-sexual orientation inventory (SOI), study has demonstrated that females who expect heavy paternal investment will try to emphasise fidelity and chastity thus scoring lower on the SOI and demonstrating a more restricted sexuality. Females who have an unrestricted sexuality will not expect a big investment and will show they are open to sexual intercourse by emphasising sexual attractiveness.
Parental investment is any effort that increases quality of offspring, this could be things such as spending of time, energy and resources to help offspring to survive. Traditionally, females have greater parental effort while males have greater mating effort, and males often compete for the access to females and their investment in order to pass on their genes. In human parental investment, males are more uncertain compared to females when investing in offspring, as they don't know if the offspring belongs to them, so called paternity uncertainty. Lack of paternal investment due to father absence constituted a unique and independent path to early sexual activity and adolescent pregnancy for females. Many studies have identified the absence of father as a major risk factor for both early sexual activity and teenage pregnancy. In the study, females that had no father absent since the age of 0 to 5 are more likely to engage in sexual activities earlier than females who had no father absent from 6 to 13.
Another study looked at psychological/social stress in the developmental environment, the age of first menarche and the age of giving first birth to offspring, the data suggests that the family environment with father absence makes the family support more uncertain, which might trigger higher stress, resulting in early menarche. As menarche is the sign of puberty, females are more likely to engage in sexual activities earlier because of the facilitation of early menarche. Unrestricted female sexuality could also be explained by females who lack paternal input would seek outside for social support due to uncertain future and uncertain availability of males.
Research indicates that lack of paternal investment also impacts on male development. Higher mortality, higher impulsivity, higher rape-prone activities and sexual coercion is associated with father absence, compared to males who grew up in father-presence environment. These findings are consistent with life-course adversity models of early sexual activity and teenage pregnancy, which stated that the stress within the family environment over lifetime provokes the earlier onset of sexual activity and reproduction.
Trivers' theory has been very influential as the predictions it makes correspond to differences in sexual behaviours of men and women, as demonstrated by a variety of research. Cross-cultural study from Buss (1989) shows that males are tuned into physical attractiveness as it signals youth and fertility and ensures male reproductive success, which is increased by copulating with as many fertile females as possible. Women on the other hand are tuned into resources provided by potential mates, as their reproductive success is increased by ensuring their offspring will survive, and one way they do so is by getting resources for them. Alternatively, another study shows that men are more promiscuous than women, giving further support to this theory. Clark and Hatfield found that 75% of men were willing to have sex with a female stranger when propositioned, compared to 0% of women. On the other hand, 50% of women agreed to a date with a male stranger. This suggests males seek short term relationships, while women show a strong preference for long-term relationships.
However, these preferences (male promiscuity and female choosiness) can be explained in other ways. In Western cultures, male promiscuity is encouraged through the availability of pornographic magazines and videos targeted to the male audience. Alternatively, both Western and Eastern cultures discourage female promiscuity through social checks such as slut-shaming.
PIT (Parental Investment Theory) also explains patterns of sexual jealousy. He found males are more likely to show a stress response when imagining their partners showing sexual infidelity (having sexual relations with someone else), and women showed more stress when imagining their partner being emotionally unfaithful (being in love with another woman). PIT explains this, as woman's sexual infidelity decreases the male's paternal certainty, thus he will show more stress due to fear of cuckoldry. On the other hand, the woman fears losing the resources her partner provides. If her partner has an emotional attachment to another female it's likely that he won't invest into their offspring as much, thus a greater stress response is shown in this circumstance.
A heavy criticism of the theory comes from Thornhill and Palmer's analysis of it in A Natural History of Rape: Biological Bases of Sexual Coercion, as it seems to rationalise rape and sexual coercion of females. Thornhill and Palmer claimed rape is an evolved technique for obtaining mates in an environment where women choose mates. As PIT claims males seek to copulate with as many fertile females as possible, the choice women have could result in a negative effect on the male's reproductive success. If women didn't choose their mates, Thornhill and Palmer claim there would be no rape. This ignores a variety of sociocultural factors, such as the fact that not only fertile females are raped - 34% of underage rape victims are under 12, which means they are not of fertile age, thus there is no evolutionary advantage in raping them. 14% of rapes in England are committed on males, who cannot increase a man's reproductive success as there will be no conception. Thus, what Thornhill and Palmer called an 'evolved machinery' might not be very advantageous.
Trivers' theory overlooks that women do have short-term relationships such as one-night stands, while not all men behave promiscuously. An alternative explanation to PIT (Parental Investment Theory) and mate preferences would be Buss and Schmitt's sexual strategies theory. SST argues that both sexes pursue short-term and long-term relationships, but seek different qualities in their short- and long-term partners. For a short-term relationship women will prefer an attractive partner, but in a long-term relationship they might be willing to trade-off that attractiveness for resources and commitment. On the other hand, men might be accepting of a sexually willing partner in a short-term relationships, but to ensure their paternal certainty they will seek a faithful partner instead.