There is no area of policy where the Government deserves more credit than childcare. The changes since 1997 have been remarkable, even if, in fact, little remarked upon. The ambitions set out in the recent pre-budget report are unfashionably grand and visionary. Unlike lots of ex tempore public policy, the strategy is based on a reputable and established body of evidence. There is now a wealth of data to support the old Jesuit hunch that a pound spent on a child early in its life has a far better return than a pound spent in the teenage years. Early spending is also pleasingly progressive: the effects are more marked lower down the income scale.

However, a note of cultural pessimism is always in order. The very ambition of the plan should give us pause for thought. In particular, it is useful to think about what might stand in the way of success. The promise of early years policy could be very great. Some of its advocates see it as the fulfilment of the scholastic dream: although comprehensive education failed to produce durable social mobility, targeted policy for young children just might do so. This could be true but there are reasons why it might not be.

Here are five reasons to be miserable.

First, some characteristics are heritable and not susceptible to policy interventions. It is not true, as we once thought, that genetic inheritance is just given. Genes, as Matt Ridley shows in his “Nature via Nurture”, are shaped by their environment but they are not endlessly mutable and our basic talents are a limitation on the capacity of policy to change our world.

Second, the influence of parents is obviously not limited to the genetic patterns they bestow. The home environment, not institutional care, is the most important determinant of child development. There is some evidence that parent education programmes improve parenting behaviour but precious little that this, in turn, improves child outcomes.

Third, a strategy based on improving life chances is fantastically costly.

Universal but low-quality childcare could help to secure an easier passage for women back into the labour market. This is a laudable aim but it is not the same thing as the desire to improve child development. That latter objective requires childcare of the very highest quality. In short, that means a highly qualified workforce. And that, in turn, means a lot of money. The Social Market Foundation and the Daycare Trust asked PricewaterhouseCooper to calculate the costs of a strategy for the early years focused solely on improving child development. To provide 12 months paid parental leave, a home care allowance for children aged between one and two, extended duration of free provision and the required workforce improvements would cost about pound;30bn, at current prices. The largest single constituent of this alarming number is the increased workforce cost, 70 per cent of the total. The only industry in which labour cost is higher as a proportion of total revenue is Premier League football.

Fourth, we as yet know very little about what happens to the gains made by effective early years policy. There is some evidence that they do begin to fade. We ought not to think of early years policy as in any way a form of inoculation. There are things we could stop doing to finance a shift to the early years - government training programmes, for example, have a negligible success rate - but the early years will not be a panacea and we ought perhaps to temper some of the wilder claims made on its behalf.

Fifth and finally, there is the dog yet to bark: the conflict between professions. This is inevitable and yet still conducted sotto voce. It will not be long before the tensions between those in education and those in care become explicit. There will be casualties in the attempt to develop child practitioners, combining several extant disciplines and arousing tribal professional jealousies. One needs only recall the furore over classroom assistants to get a taste of the row to come over who does what and when.

Phil Collins is director of the Social Market Foundation