It could be that while having children does indeed make them happier people, there's something else going on at the same time that makes them feel terrible. It would be stress. Mark Wooden and Hielke Buddelmeyer from the Melbourne Institute have quantified it in a new study written with a leading United States economist Daniel Hamermesh called The Stress Cost of Children. They looked at two types of stress: financial, and being short of time. After examining data from surveys in Germany and Australia they found that financial stress wasn't that important. Time stress was critical. To measure it in Australia the parents were asked: "How often do you feel rushed or pressed for time?" In Germany they were asked: "Think about the last four weeks. How often during this period did it happen that you felt rushed or under time pressure?" In Germany, the birth of a child added enormously to the mother's time stress and not at all to the father's.

In Australia, both parents became more stressed, but for the mother the effect was three times as big. Worse, it never really vanished. Whereas whatever financial stress a child brought disappeared quickly, the time stress for women continued for at least four years – the limit of the survey. But how much stress? The researchers used complex econometric techniques to answer the question: what transfer of earnings from the father to the mother would be needed to reduce the mother's financial stress by an amount equal to the increased time stress generated by the birth. The answer shocked them. (They are all men by the way.)

In Australia the transfer would need to be more than twice as much as the father earned. "Clearly, there is no reasonable transfer of earnings from husband to wife that can compensate for the increased time stress that she experiences with the new child," they concluded. "These simulations suggest that the psychological cost of a new child is huge in comparison to the monetary cost. While other simulations would generate different monetary comparisons to the time stress experienced by new mothers, given our estimates it is doubtful that any reasonable simulation would suggest that these costs are small." The extra stress is all the more shocking because before birth new mothers are likely to be unusually free of stress. It dips before birth and then soars. The report says that might be because parents pick times of unusually low stress in which to have a child. It might also be because women are more fertile when they are less stressed. At the other end of childhood emptying the nest might be expected to dramatically lower stress in the same way that gaining a child raises it. But their study of a different group of parents whose children left home found what they called an asymmetry. Departures eased stress only gradually. Stress began subsiding four or more years before the child left and then kept falling at roughly the same rate in the years that followed. It's as if the child never really left, or as if the child began withdrawing before they said goodbye.

Wooden and Buddelmeyer found that having children generated "a permanent lifetime increase in perceived stress". Which doesn't necessarily mean it's a bad thing. There are upsides from having children as downsides: pleasure, a feeling of importance and a feeling of contributing to humanity are among them. But they are likely to be forever offset by a feeling of being tired, anxious or short of time. None of these are reasons not to have children. But they are reasons to think about it carefully. Once made the decision can't be undone. And I don't know of many parents who would like to undo it. It just means that when parents tell you how wonderful it is, they're not giving you the full picture. Peter Martin is economics editor of The Age.