SEVEN BILLION PEOPLE: I had better write fast. Sometime between my deadline to submit this story and the time it goes live, the estimated world population will exceed 7 billion for the first time ever.

As I stare at the population clock, I am paralysed at the sheer speed at which the number of people grows. I am terrified at how our world might support all those lives.

But the biggest challenge of all is how to elevate the lives of more than one billion people already alive who eke a living from less than $1 per day, so that they live a life free of famine and preventable disease.

Since at least 1798, when Thomas Robert Malthus argued that population would soon outstrip agricultural production, pessimists have foretold famine, disease and conflict if population growth isn’t reined in.

But some economists and demographers don’t see the problem this way. To them, Malthus was a crank who never grasped the ambit of human ingenuity. Industrialisation, slave-powered Caribbean sugar colonies and the New England cod fisheries revolutionised food production in the 19th Century. Green revolution supercrops staved off Malthusian misery in the 1960s.

Yet we need only look at the appalling famines in Somalia and neighbouring countries to see what happens when too many people try to scrape a living from the land. The great biologist EO Wilson puts it sharply: “The raging monster upon the land is population growth. In its presence sustainability is but a fragile theoretical construct. To say, as many do, that the difficulties of nations are not due to people but to poor ideology or land-use management is sophistic.”

Longevity

It took 2 million years of human history for humanity to notch up its first billion in 1800. Yet the next billion took only 127 years, and by 1960, a mere 33 years later, there were three billion. The fastest ever growth rate came in the sixties, with the fourth billion taking only 14 years.

But despite this explosion, the world population growth rate has slowed dramatically since the early seventies. To me, this slowing in global population growth is the big story. It is where any hope for a sustainable future lies.

For most of history, our ancestors had many children, yet population grew at a trickle because life tended to be short. Mothers routinely died in childbirth, and infants and young children often didn’t make it to adolescence.

The explosion in human population, from 1 billion at the start of the industrial revolution to 7 billion barely 200 years later, comes almost entirely from improved survival.

A genuine understanding of hygiene and disease, immunisation programs, sanitation, clean water, antibiotics and massive improvements in agriculture all contributed to longer lives and better childhood survival.

Whenever mortality plummets like this and birth rates remain high, then population growth goes through the roof. Our capacity to breed prolifically should be no surprise. After all, evolution has equipped us to excel at reproduction.

Quality over quantity

Every person alive today comes from an unbroken line of successful ancestors – people who managed to have at least one child. Many of the most successful ancestors in history are the people who had large numbers of children.

As a result, the genes we inherited from them tend to be genes that give us the behaviour, physiology and anatomy of successful breeders.

But evolution can also be subtle. Sometimes the best way to become an ancestor is not to go at it like rabbits, but to be more judicious in how much we invest in each of our children.

People invest enormous effort in caring for their children, teaching them and preparing them for the day they have to make their own way in the world.

In short, millions of years of evolution have equipped us to be exquisitely sensitive to our circumstances in our decisions about how much to invest in each of our children.

When mortality – especially child mortality – is high, it makes sense to have plenty of kids, because not all of them will survive. More so when children can gather food or work on the farm.

But when child mortality drops and skills and knowledge become economically more rewarding than manual labour, then the best way to ensure each child’s success is to invest in caring for them and educating them.

That is precisely what happens when economies industrialise. Families that educate and invest in their children achieve social mobility.

Since the start of the industrial revolution, birth rates have plummeted in Europe, north America and Australia as families shifted from having as many children as they could afford to investing as much as they could in a modest brood.

Sexual conflict

There is one more important but often-overlooked piece of this puzzle. Until now I have assumed that families operate in harmony – that what is good for the mum is equally good for the dad and for the kids.

But in evolutionary terms, different family members can have disturbingly different agendas.

Every baby a woman has increases her chances of dying in labour and worsens her likely long-term health. While a father may be bereft at losing his partner in childbirth, he doesn’t lose everything. He can always remarry.

So women often do best, in evolutionary terms, if they have fewer, high quality offspring quality. For men, evolutionary success is more of a numbers game and men often want more children from their wives and more chances to have extra children from affairs.

At the heart of the trade off between offspring quality and quantity is an often sub-conscious tension between husbands and wives, and between men and women within societies.

The industrial era also brought forth feminism, and every step in the empowerment of women shifted the battle over family planning toward quality, smaller, families.

Safe contraception and access to abortion give women the means to limit their fertility.

Women’s education and employment give them knowledge and power within the home to do so. And when women can earn a good wage, families that limit their fertility enjoy more time with two earners.

Michelle Goldberg concludes her wonderful book The Means of Reproduction: Sex, Power and the Future of the World by arguing that only when governments take women’s needs seriously will we have any chance of avoiding Malthusian misery. But we should take those needs seriously anyway, because individual women are important.

That is why, she concludes, “There is no force for good as powerful as the liberation of women.”

This article is based on a chapter from Rob Brooks' book Sex, Genes and Rock and Roll: How evolution has shaped the modern world.

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