Because human beings reproduce sexually, each child is a vehicle for only 50% of a parent’s genes; whereas the child is a vehicle for 100% of its own genes. This creates conflict between parent and offspring because each of the children are equally valuable to the parent, but each child is more important to themselves than to their siblings.

It is in the parent’s genetic interest to share their food, time and effort between the children to ensure they survive and reproduce; however, it is in the child’s interests to obtain as much of these resources as possible for itself.

Siblings do share 50% of their genes with each other, so it is in the interests of the child to help its siblings genes survive, however, their own needs are twice as important as they share 100% of their genes with themselves (Trivers, 1974).

Resource Allocation

It is in the survival interests of the parent’s genes to vary the amount of resources given to each child according to the circumstances. The following factors have been found to be important:

Age of parent

This will affect how many children the parent can expect to have in the future.

Age of children

Younger children need more resources, so parents tend to allocate more time and effort to them than older children.

Health of children

Sick children need more resources than healthy children; however, there will be a cut off point where it is no longer worth investing (see post natal depression).

Weaning conflicts

Weaning conflicts are an example of a conflict over resource allocation. Weaning is when babies stop relying on the mother’s milk and move on to other food. Parental investment by breastfeeding enhances the reproductive fitness of the infant and, therefore increases the probability of the long term survival of the parent’s genes. This is because breast milk improves the health of the immune system; can improve IQ scores (Caspi et al. (2007); reduces the risk of type 1 and type 2 diabetes, obesity, allergies and asthma; provides protection against respiratory and intestinal infections; reduces the risk of sudden infant death Syndrome; and reduces the risk of the child developing breast cancer in later life.

Although the benefits of breastfeeding diminish with age, it can continue to be beneficial to the health of a child up to the age of seven. In many cultures, it is normal for breastfeeding to last for 2-4 years. Nevertheless, regular, repetitive nipple stimulation inhibits females’ menstrual cycles for up to three years after giving birth (Short, 1987). This means that women are less likely to become pregnant while they are breastfeeding (Birdsell, 1979). This means that the benefits to the child come at a reproductive cost to the mother.

According to Trivers (1974), because the mother places equal value on each offspring, whereas the child values themself twice as highly as each of their siblings, the mother will want to wean the child when the benefits the child is obtaining from the milk are less than the benefit a younger sibling would obtain, while the child will want to wean itself when the benefits it is obtaining are less than half of the benefit a younger sibling would obtain.

This means that the mother will always want to wean the child before the child would like to be weaned. Trivers (1981) goes on to claim that it is in the child’s interests to use distress signals (e.g., crying, tantrums…) to secure additional investment from the parent.

Sacrifices

At first glance, it does not make evolutionary sense that an organism would sacrifice itself to save another, since any genes that predispose an animal to sacrifice itself would surely be quickly wiped out by natural selection. However, according to William Hamilton, in certain circumstances it is adaptive to sacrifice oneself for others. Hamilton’s Inequality states that self sacrifice is advantageous when Br > C, where B = the benefit in terms of reproductive fitness in the beneficiary of the altruistic act and C is the cost in terms of reproductive fitness to the altruist and r = the degree of relatedness.

Example: if a parent were to sacrifice itself to save a single offspring it would cost all of its reproductive fitness, but 50% of the parent’s genes would be saved in the offspring (r = 0.5 in this case). Thus, in this case, self sacrifice would not be adaptive. If the parent were to sacrifice itself to save two children, however, there would be no net benefit because the parent would be sacrificing 100% of its genes and saving 100% in the offspring. On the other hand, a parent sacrificing its life to save 3 children would preserve 150% of its genes and this would, therefore, be adaptive.

Parent-offspring conflict occurs over making sacrifices because the optimal conditions for the survival of the parent’s genes are different to those of the offspring. For the parent, if any of the offspring make a sacrifice where the benefit to a sibling is greater than the cost to themselves it is advantageous. However, from the offspring’s point-of-view it is only advantageous if more than 2 siblings benefit overall. This means that parents will encourage their offspring to make sacrifices for their siblings, but the offspring will be reluctant to comply as it would not be in the interests of their genes.

Parent-offspring conflict and Postnatal Depression

Most women develop the ‘baby blues’: mild feelings of depression and periods of weeping within a few days of giving birth (Pitt, 1973); however this usually passes after about a week. A smaller percentage of women (between 10 and 15%) experience the full clinical syndrome of postnatal depression, which is more severe and long lasting. A very small minority of women (about 1 in 1000) develop a very severe disorder called puerperal psychosis, the symptoms of which include severe depression, hallucinations, confusion and delusions. According to Daly and Wilson (1995), however, the content of the delusions are not as bizarre as they are in other psychotic conditions, they are generally focused on the baby, for example, fear that they will not be able to care for the baby adequately, guilt about thoughts of killing the baby and fear of not loving the baby enough.

Some theorists have proposed that postnatal feelings of depression serve an evolutionary purpose of assessing whether parental investment in the infant is worthwhile.

Sex Differences in Parental Investement

Evolutionary Explanations of Parental Investment