Claims that Planned Parenthood clinics have sold the organs and tissues of unborn children have caused widespread revulsion, and re-opened the debate about abortion. This debate is usually phrased as ‘pro-life’ versus ‘pro-choice’, which is an question of ethics. But I believe behind this lies a question of psychology: how much do we love and want children?

The evidence is that in recent times, we want them less and less. One hundred and fifty years ago, when abortion was a despised and criminal act, populations were growing fast. Today, with abortion legal and supported by government, the birth-rate has dropped below replacement level in every developed country except Israel. But the fall in birth-rate started more than a century ago, long before the legalization of abortion or even the invention of the Pill. The rise of abortion is best seen as a result of the fact that we do not value children as much as we used to.

As a society we lean towards economic explanations for behaviour, and in some ways this makes sense. People are guided by money in their choice of job. Thus, while tax accountant or actuary might not be most people’s first career choice, society manages to find enough of them to fill its needs. Women also tend to be drawn to men with money, which is why Cinderella wanted to marry the prince rather than the local dustman.

But money has less impact on family size. In fact, wealthy societies tend to have fewer children than poorer ones, and this is not just a feature of the modern world. In late Republican Rome, the birth-rate plummeted while Romans enjoyed the wealth and luxury gained by conquest. The Emperor Augustus savagely scolded  his nobility for their failure to breed (with little effect, it must be said). In Imperial China, wealthy scholar-gentry families tended to decline after two or three generations, both through loss of work ethic and from having too few children.

So why have people ceased wanting children? Laboratory research may provide the answer.  Over the past eight years my research team has been investigating the effects of mild food shortage on rats. This has a wide range of effects on physiology and behavior, such as making them more active and exploratory. They also, and very significantly, become more interested and effective mothers, spending more time with their young and doing much better at gathering them back into the nest.

Our research suggests that the most important age for this effect is late childhood and adolescence, basically from age 6 to 18. An illustration of this is the effects of the Great Depression, a distinct period of blight in a century of growing prosperity. The people who passed late childhood and adolescence in this period became the parents of the baby boom, from the late 1940s to the early 1960s.

Fortunately, science suggests that economic blight is not the only way to achieve this result. Restricting sexual activity has a similar effect to food shortage, such as in reducing testosterone. The same applies, we believe, to a number of other behaviors associated with religion. Thus it is that people who faithfully follow demanding religious traditions tend to have more children. Examples from the modern world include Mormons and Hasidic Jews. Utah, for example, has the highest birth rate of any American state, well above replacement level.

Faith has always been linked to love of children. Nothing more enraged the prophets of Israel than the sacrifice of children to idols. Jesus loved children, and famously rebuked those who would keep them away from him. The early Christians rescued and raised babies that had been abandoned at birth, a common practice in the ancient world and not considered illegal or even wrong. In fact, the Spartan state might demand that parents expose a newborn child, if it did not appear strong and healthy.

Our society makes the killing of babies a criminal act, while the killing of unborn children is merely a medical one. Many people might consider this distinction to be quite arbitrary. But either action is less likely if we value children more

The most obvious way to achieve this would be through a revival of religious faith and practice. But science might be able to provide some assistance by immunizing people against the bad effects of too much wealth. The experiments of my research team have shown that it is possible to restore healthy maternal behavior in rats without making them hungry.

I started having children while working as a gardening contractor, supporting a wife and in rented housing, with barely a cent in the bank. The reason was that I loved children and wanted a family as soon as possible. To date I have ten. Children are, to me, an enormous joy, which no new car or expensive holiday could match. We have children because we want them.

Dr. Penman, author of Biohistory: Decline and Fall of the West, has devoted his life to the scientific understanding of social change.