Physics and economics are bedevilled by the phenomenon known as ''hysteresis''. It's where, once made, a change can't be easily undone. In the physical world it applies to breaking an egg. In the world of social policy it applies to extending to the well-off benefits that were previously reserved for the poor.

The Baby Bonus is a case in point. By the end of the Hawke-Keating government in 1996 Australia had one of the tightest social security systems in the world. On one estimate, 92 per cent of government payments went to the half of the population that earned the least. By the end of the Howard government a decade later, many more of the payments were spilling over to the top half. A lesser 87 per cent went to the bottom half.

The Australian National University's Professor Peter Whiteford, who did the calculations, is keen to point out that even after the Howard era, Australian payouts were still the most tightly aimed of the nations that make up the Organisation for Economic Co-operation and Development.

The Baby Bonus was one of Howard's most brazen spillovers. Instead of directing it to parents who had earned the least, it was funnelled to those who had earned the most. Mothers who had high incomes in the year before their child was born were able to claim $2500 per year. Mothers who had been on low incomes got $500.