Leading article: A big idea – but will these tiny sums really help?

After months of posturing from Downing Street on subjects ranging from terrorism to the oil price, it is refreshing to see Gordon Brown engaging with the sort of issue that brought him into politics in the first place: social mobility. In a speech yesterday, Mr Brown called it: "The greatest test of our time – to build a fairer, more prosperous and upwardly mobile Britain". He is right about the fundamental moral importance of this issue. It is a scandal that, despite decades of economic growth, the best predictor of a child's future status and well-being in 21st century Britain is the wealth of the family they are born into. Nor can this inequitable situation be pinned on unfortunate shifts in the global economy. A study by the Sutton Trust last year found that, along with the United States, Britain has one of the lowest levels of social mobility in the developed world.

Mr Brown is also right about the strategic political importance of the issue. If he does not assert ownership of it, David Cameron will happily fill the gap with his compelling analysis of Britain's "broken society". Yet the Prime Minister's analysis of how we came to this pass is deficient. In his speech yesterday, he argued that the great post-war gains in social mobility stalled in the 1970s and 1980s leaving a "lost generation" of "Thatcher's children". This partisan framing of the history is a mistake. Failures in the state education system – one of the root causes of stalled social mobility – are a responsibility of both Labour and Conservative administrations.

It was Labour who pushed through the flawed comprehensive secondary school model in the 1960s and 70s. And Margaret Thatcher watched hundreds of grammar schools close during her time in power. For all their manifest faults, it was these schools that had propelled a generation of modestly-born children into positions of power and influence. Mr Brown should know all about this; he was one of them.

In fairness to the Prime Minister, he has done some important work since 1997 in levelling the playing field through his tax credits and "baby bonds". But his approach has been too focused on income inequalities. Parental poverty of ambition is just as damaging to a child's life chances.

Both parties have also ignored the fact that to have upward social mobility, it is also necessary to have some level of downward social mobility. The Tories' populist attack on inheritance tax was stolen wholesale last autumn by Mr Brown, panicked by the debacle over the non-election. Yet what more pernicious means of cementing income divisions in society exists than hefty untaxed inheritances?

Mr Brown's big idea yesterday was a Child Development Grant. Parents in deprived communities will be given £200 grants in return for taking part in programmes to improve the "health, well-being and social development" of their children. This is based on schemes in the US that have targeted the most "hard to reach" families. This approach should not be dismissed. Early-years intervention is an intelligent focus, considering all the research that indicates a child's life chances are often determined by the age of five. Yet the sums here are so small that it is hard to see it having any significant effect in eroding inequalities.

The uncomfortable reality is that Britain is treading water, while other nations are swimming far ahead. It will take a sustained assault by government on the causes of inequality, across education, welfare and taxation, to deliver the type of fair society that all the political parties claim to want. Tinkering has not worked. Now is the time to think big.