I have two great kids and have always been keen on a third. But my partner thinks enough is enough. My long campaign to change her mind has been to no avail. Barring a Biblical-scale miracle, we're done.

One of the biggest decisions any couple will make is whether to have children and, if so, how many. But as Australia's population ages, the way mothers and fathers bargain over babies is increasingly important for the whole society, not just individual families.

It's more than a decade since Peter Costello set the demographic alarm bells ringing with his famous call for Australian couples to have "one for the father, one for the mother and one for the country". The former treasurer made those remarks when the Howard government introduced its baby bonus in 2004 amid warnings about the long-term consequences of our ageing population. The fertility rate did rise between 2004 and 2008, although demographers question whether government policy had much to do with it.

But from a peak of 2.02 in 2008, the fertility rate has fallen back to 1.8, the latest figures show - not much higher than when Costello urged couples to breed for the nation. Finding ways to sustain a healthy birth rate is as important now as it was when Costello was treasurer.