The bond between parents and their children is often regarded as something loving, caring, and supportive. However, in evolutionary biology, we see this bond more as battle. To explain this, we have to visit the groundbreaking work of two evolutionary biologists: the late William Hamilton and Robert Trivers. Hamilton first introduced the notion of inclusive fitness (total fitness including kin such as offspring and relatives) to studies of altruism. Specifically, he proposed that a gene for altruistic behaviour can spread if the cost of the altruistic behaviour is equal to or less than the benefit gained by the recipient, while taking into account the relatedness between the actor and recipient. This came to be known as Hamilton’s rule, and it singlehandedly altered the way many researchers view social behaviours.
A decade later, Trivers recognized that Hamilton’s rule could partially explain the social dynamics between parents and their offspring. Focusing on relatedness, Trivers proposed that the fitness interests of parents and their offspring should differ because the offspring are twice as related to themselves than to their parents. This asymmetry in relatedness, now known as parent-offspring conflict, should lead to differences in parental investment, with the offspring demanding twice as many resources than the parent is willing to provide. Support for parent-offspring conflict has been numerous and comes from both theoretical models and empirical studies. One interesting theory is that of the kinship theory of genomic imprinting. Genomic imprinting occurs when maternal and paternal alleles are expressed differently in the offspring. The kinship theory explains the evolution of genomic imprinting by using parent-offspring conflict as a platform. It focuses on parental investment on a genetic level; specifically, why and how are maternal genes expressed in the offspring versus paternal ones? Differences in gene expression are expected when the fitness interests of the mother and father differ. One key example would be in the case of polygamy, where the mother is equally related to all the offspring but the father is only related to his offspring and not those sired by other males.
Recently, researchers have started to take a more applicable approach to imprinting by exploring the link between genetic conflicts and human diseases. Given the intimate contact between the mother and her (unborn) child in the womb, it is no surprise the kinship theory has been associated with complications in pregnancy (an idea first proposed by evolutionary biologist David Haig). Aside from pregnancy, genomic imprinting has been linked to a number of behavioural, cognitive, and/or developmental problems after birth. More recently, there has even been a shift towards studying genetic conflicts in the brain. Bernard Crespi, an evolutionary biologist at Simon Fraser University, is starting to explore the role of genomic imprinting on mental disorders, including autism, schizophrenia, bipolar disorder, and depression.
Since the days of Hamilton, we have made great strides in our evolutionary understanding of how genes, albeit selfish ones, can cause rifts between the mother versus offspring and mother versus father. With potentially a few hundred imprinted genes in the human genome, perhaps we should think of the bond between parents and their offspring as a battle, just like we do in nature.