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Parental investment in humans
Humans have evolved increasing levels of parental investment, both biologically and behaviorally. The fetus requires high investment from the mother, and the altricial newborn requires high investment from a community.

Maternal Investment
Trivers (1972) hypothesized that greater biologically obligated investment will predict greater voluntary investment. Mothers invest an impressive amount in their children before they are even born. The time and nutrients required to develop the fetus, and the risks associated with both giving these nutrients and undergoing childbirth, are a sizable investment. To ensure that this investment is not for nothing, mothers are likely to invest in their children after they are born, to be sure that they survive and are successful.

Hemochorial placentation
The developing human fetus––and especially the brain––requires nutrients to grow. In the later weeks of gestation, the fetus requires increasing nutrients as the growth of the brain increases. Rodents and primates have the most invasive placenta phenotype, the hemochorial placenta, in which the chorion erodes the uterine epithelium and has direct contact with maternal blood. The other placental phenotypes are separated from the maternal bloodstream by at least one layer of tissue. The more invasive placenta allows for a more efficient transfer of nutrients between the mother and fetus, but it comes with risks as well. The fetus is able to release hormones directly into the mother’s bloodstream to “demand” increased resources. This can result in health problems for the mother, such as pre-eclampsia. During childbirth, the detachment of the placental chorion can cause excessive bleeding.

Relative to most other species, human mothers give more resources to their offspring at a higher risk to their own health, even before the child is born. This is associated with the evolution of a slower life history, in which fewer, larger offspring are born after longer intervals, requiring increased parental investment.

Obstetrical dilemma
Humans have evolved both bipedalism and large brain size. The evolution of bipedalism altered the shape of the pelvis, and shrunk the birth canal at the same time brains were evolving to be larger. The decreasing birth canal size meant that babies were born earlier in development, when they have smaller brains. Humans give birth to babies with brains 25% developed, while other primates give birth to offspring with brains 45-50% developed. A second possible explanation for the early birth in humans is the energy required to grow and sustain a larger brain. Supporting a larger brain gestationally requires energy the mother may be unable to invest.

Species whose newborn young are unable to move on their own and require parental care have a high degree of altriciality. Human children are born unable to care for themselves and require additional parental investment post-birth in order to survive.

Birth attendance and sociality
The obstetrical dilemma makes birth challenging, and a distinguishing trait of humans is the need for assistance during childbirth. The altered shape of the bipedal pelvis requires that babies leave the birth canal facing away from the mother, contrary to all other primate species. This makes it more difficult for the mother to clear the baby’s breathing passageways, to make sure the umbilical cord isn’t wrapped around the neck, and to pull the baby free without bending its body the wrong way.

The human need to have a birth attendant also requires sociality. In order to guarantee the presence of a birth attendant, humans must aggregate in groups. It has been claimed that humans have eusociality, like ants and bees, in which there is relatively high parental investment, cooperative care of young, and division of labor. It is unclear which evolved first; sociality, bipedalism, or birth attendance. Bonobos, our closest living relatives alongside chimpanzees, have high female sociality and births among bonobos are also social events. Sociality may have been a prerequisite for birth attendance. Bipedalism and birth attendance could have evolved as far as five million years ago.

Grandmother hypothesis
As female primates age, their ability to reproduce decreases. The grandmother hypothesis describes the evolution of menopause, which may or may not be unique to humans among primates. As women age, the costs of investing in additional reproduction increase and the benefits decrease. At menopause, it is more beneficial to stop reproduction and begin investing in grandchildren. Grandmothers are certain of their genetic relation to their grandchildren, especially the children of their daughters, because maternal certainty of their own children is high, and their daughters are certain of their maternity to their children as well. It has also been theorized that grandmothers preferentially invest in the daughters of their daughters because X chromosomes carry more DNA and their granddaughters are most closely related to them.

Paternal investment
As altriciality increased, investment from individuals other than the mother became more necessary. High sociality meant that female relatives were present to help the mother, but paternal investment increased as well. Paternal investment increases as it becomes more difficult to have additional children, and as the effects of investment on offspring fitness increase.

Concealed ovulation
Women can only get pregnant while ovulating. Humans are unique among primates in that ovulation is concealed, or not signaled externally. Concealed ovulation decreases paternity certainty because men are unsure when women ovulate. The evolution of concealed ovulation has been theorized to be a result of altriciality and increased need for paternal investment. There are two ways this could be true. First, if men are unsure of the time of ovulation, the best way to successfully reproduce would be to repeatedly mate with a woman throughout her cycle, which requires pair bonding, which in turn increases paternal investment. The second theory states that decreased paternity certainty would increase paternal investment in polygamous groups, because more men may invest in the offspring. The second theory is better regarded today, because all mammals with concealed ovulation are promiscuous, and men display relatively low mate-guarding behavior, as monogamy and the first theory require.

Sociosexuality
A reproductive tradeoff exists between the number of children had and the investment in each child. The more children had, the fewer resources can be invested in each child.

Sexual dimorphism
Sexual dimorphism is the difference in body size between male and female members of a species as a result of intrasexual selection, which is sexual selection that acts within a sex. High sexual dimorphism and larger body size in males is a result of male-male competition for females. Primate species in which groups are formed of many females and one male have higher sexual dimorphism that species that have both multiple females and males, or one female and one male. Polygynous primates have the highest sexual dimorphism, and polygamous and monogamous primates have less. Humans have the lowest levels of sexual dimorphism of any primate species, indicating that we have evolved decreasing levels of polygyny. Decreased polygyny is associated with increased paternal investment.

The Demographic Transition
The demographic transition describes the modern decrease in both birth and death rates. From a Darwinian perspective, it does not make sense that families with more resources are having fewer children. One explanation for the demographic transition is the increased parental investment required to raise children who will be able to maintain the same level of resources as their parents.