Maternal instinct and biology: evolution ensures we want sex, not babies

Many women hear an ominous ticking of their "biological clock" when they reach their 30s, while others never hear it at all.

Some believe the compulsion to bear babies is biologically inbuilt – even suggesting women who refuse their supposed evolutionary duty are being selfish.

Others hold the view that this so-called "maternal instinct", also referred to as "baby fever", has nothing to do with biology and is a social construct.

It's unhelpful to explore this debate through a strictly dichotomous "nature vs nurture" prism. Both biology and culture likely contribute to our reproductive behaviour.

Reproduction doesn't require any "inherited" preference to have children, since natural selection already favours mechanisms that result in reproduction, most significantly through the sexual urge.

But that version of the maternal instinct that relates to a mother's ability and need to nurture and protect her child may indeed be hardwired, facilitated by the release of certain hormones and other necessary biological changes.

Sexual urge

The exquisite diversity of past and present lifeforms comes from a single critical feature – reproduction.

Individuals genetically disposed to be indifferent to sex will theoretically be selected out of the population, in favour of those with a greater commitment.

This is a self-evident feature of the evolutionary process.

Imagine a population of people or animals who enjoy sex, where that enjoyment has a genetic basis. This would determine their reproductive success. Now introduce into this population those genetically predisposed to be sexually inactive.

These sexually inactive individuals will not produce offspring, so there will be no sexually inactive individuals in the next generation.

In other words, a genetic disposition to avoid sex will neither become established nor maintained.

Some argue the so-called "biological clock", triggering an enhanced awareness of reproduction among childless women in their 30s, is natural selection at work. Maybe.

There is some evidence that fertility decisions may have a genetic basis. For instance, studies that looked at the age of first attempt to have a child in Finnish populations showed children had similar patterns to those of their parents.

But these only proved there is a genetic influence for when women decided to have a child, rather than whether they decided to at all.

We are notoriously susceptible to the influence of others (witness the broad success of advertising and, one hopes, education).

So, like many other aspects of human behaviour, it remains unclear whether the strong longing for a child – "baby fever" – is driven by our genes or is a social construction.

Defying biology

Until recently, sex and reproduction were inextricably entwined in all organisms. The discovery of contraceptive technology severed that nexus for one species.

With varying reliability, humans can now have sex without having babies. So in terms of biological evolution, a genetic preference for sexual activity is no longer equivalent to a maternal (or paternal) instinct to have offspring.

There are many women in our society who aren't interested in having children.

For instance, the number of US women between 34 and 44 who have never had children has increased by around 10% since 1976. And a survey of more than 7,000 Australian women between 22 and 27 years found nearly 10% didn't want children.

My guess is that childless women aren't necessarily sexually inactive – as natural selection likely dictates. But there may be little opportunity for selection to act on their personal choice.

It's an impressive example of human behaviour defying biological evolution. But culture and technology have immunised humans from many selection pressures. Clothing, for example, allows us to inhabit cold environments unsuitable even for naturists.

Sex isn't one of them though. Indeed, most cultures express more than a passing interest in sex – from the widespread inclusion of fertility rites in ancient societies to the almost unseemly obsession with sex in contemporary television advertising campaigns.

Nurturing instinct

In many cases, successful reproduction requires care of the developing offspring. This is often, but not exclusively, undertaken by the mother.

Nurturing offspring is then a form of "maternal instinct", as distinct from "baby fever". And nature has built in biological mechanisms to ensure this.

For mammalian mothers, a demanding infant stimulates the release of the hormone oxytocin, which in turn triggers a flow of milk.

Oxytocin is also implicated in a suite of maternal behaviours throughout pregnancy, strengthening a mother's bond to her fetus, which impacts on the fetus' development.

The crucial, instinctive, nurturing response to feed the child, through the release of oxytocin, occurs only during pregnancy and after birth – otherwise the hormones don't kick in.

For instance, virgin mice given oxytocin injections could learn to hear and respond to distressed calls of pups, something they were unable to do before the injections.

So it could be argued that the "urge" to have and nurture children is only ensured biologically through the urge to have sex, while the nurturing instinct is biologically inbuilt.

The so-called "biological clock", then, may be ticking to a social key.

Following reader suggestions, the lead image on this article has been changed.

Mark Elgar, Professor of Evolutionary Biology, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.