How many children are enough? How many are too many? People’s feelings about their childhood, their partners and their jobs all shape these intimate choices. Yet in aggregate, clear patterns emerge. One of the most personal decisions a couple can make has profound repercussions for their country’s future, when it may strain resources or reduce the labour force supporting an ageing population.

This is the most common reason, though not the only one, for governments to intrude into the sex lives of their citizens. They may do so coercively, though measures such as forced sterilisation are banned under international law. They may seek to encourage procreation: Mongolia’s “First Order of Glorious Motherhood” is awarded to women who have borne six children or more.

Reports emerged last month that China is considering scrapping all limits on family size – after a more modest shift from the “one-child” policy to a universal two-child rule three years ago failed to produce the baby boom officials had sought. Though birthrates have fallen dramatically in many Asian countries, Beijing’s strict birth control policies played a major part in turning the demographic dividend, which fuelled its economic miracle, into a demographic timebomb.

Meanwhile, Egypt has declared “two is enough” as it struggles to hold back a rising birthrate. The country’s president, Abdel Fatah al-Sisi, has placed population growth on a par with terrorism as the greatest danger to the country: there are almost 105 million Egyptians already, and on current trends there will be 23 million more by 2030. Egypt’s slogan mimics Singapore’s “stop at two” edict in the 1970s. How much of a role the campaign played in cutting family sizes is debatable; social and economic changes certainly had a bigger impact. But the birthrate in Singapore plummeted from 4.85 in the 1960s to 1.24 in 2015. As the government now attempts to reverse course, it has tried everything from offering cash payments for each baby and prioritised access to government-built flats to issuing pheromone-laced fragrances and arranging speed-dating events.

China isn’t handing out perfume yet. But it too is discovering that the precise factors shaping the birthrate are both hard to pin down and predict, and slow to respond to government diktat. Punishments and benefits don’t always produce the results anticipated. Handouts don’t always do much to address the broader concerns about the future, or the sense of stress and pressure in daily life, which can discourage people from having more babies. In the US, births are at a record low despite a growing number of women of childbearing age: the decline began in 2008, the year of the financial crisis.

Population matters, but panics about both low and high levels tend to be exaggerated. The former might be better addressed through measures such as immigration and increased productivity than telling people to make more babies. In the latter case, the education and empowerment of women, improved access to contraception and family planning advice (the central pillar of the Egyptian campaign) and social support (so that children aren’t the only pension and health plan on offer) are all part of the solution.

Still, Egypt might want to proceed with care. As China and Singapore have found, an apparently desirable fall in the birthrate can have unanticipated consequences. Governments are able, in the last resort and at great human cost, to enforce maximal limits. Making people have more children is trickier.