You wouldn’t think it to look at our skyrocketing global population, but many parts of the world are experiencing serious falls in fertility. A country’s fertility rate is the average number of children born to a woman over her lifetime. In most developed countries, it needs to be 2.1 or higher if the number of newborns is to compensate for citizens who die. In developing countries, where death is a more frequent visitor, this replacement threshold is even higher.
The problem is that declining fertility is intimately linked with a country’s economic and social development. As a result, more than half of the world lives in areas where fertility rates have fallen below this crucial threshold. It’s the same situation in the UK, Australia, Japan, China, Brazil, Russia, Canada and more. Some believe that these processes are irreversible, with increasing prosperity inevitably leading to diminished emphasis on childbirth.
But Mikko Myrskyla from the University of Pennsylvania thinks differently. He has found that the most developed countries have actually reversed their falling fertility rates, possibly by improving gender equality and making it easier for women to raise families while enjoying successful careers. The result is a graph that looks like a reverse tick, with a small upturn in fertility rate that only becomes evident when looking at data from the dawn of the 21st century. At the most advanced stages of development, it seems that babies make a comeback.
You could argue that life is all about cheating death and having enough sex to pass on your genes to the next generation, as many times as possible. From this dispassionate viewpoint, human reproduction is very perplexing for our reproductive potential has an early expiry date. At an average age of 38, women start becoming rapidly less fertile only to permanently lose the ability to have children some 10 years later during menopause.
From an evolutionary point of view, this decline is bizarre. Other long-lived animals stay fertile until close to the end of their lives, with elephants breeding until their 60s and the great whales doing so in their 90s. In comparison, a human woman is exceptional in losing her child-bearing potential years or decades before losing her life. Even in hunter-gatherer societies that lack our access to modern medicine and technology, women who pass through menopause can expect to live well into their sixties.
Now, a pair of scientists have proposed a new model to explain the origins of menopause. Michael Cant from the University of Exeter and Rufus Johnstone from the University of Cambridge suggest that the loss of fertility helps to lessen reproductive conflicts between successive generations of women.
A few theories have already been put forward to resolve this conundrum. I’ve previously blogged about one of these, which suggests that the menopause reduces the health risks that repeated childbirth brings to both mother and child. This idea complements the most popular theory, known as the “grandmother hypothesis“, which suggests that older, infertile women can still boost their reproductive legacy by feeding, teaching and caring for their existing children and grandchildren.
The basic idea makes sense and while some studies have backed it up, it’s clearly not the whole story. Some analyses of hunter-gatherer populations have found that the indirect advantages of helping your family don’t outweigh the potential benefits of having more children yourself. Alone, the grandmother hypothesis can explain why women continue to live past the menopause, but not why they go through it in the first place.