During a recent playdate, one of my son's white four-year-old friends looked up from Thomas the Tank Engine and pointed out the obvious. "You're black," he told my son. As a parent, these have never felt like particularly teachable moments. Toddlers have plenty of time ahead of them to acquire anxieties, affiliations and attitudes about race. But what they see primarily at their age is not race but difference - a fact that need prompt neither denial nor panic, rebuke nor rectification, unless some derogatory meaning is attached to that difference.

When my son looks to me for a cue, my aim is not to interrogate or chide but to acknowledge and deflect. In the past, I have said: "And what colour are you?" or "And you are white". But this time new material came to mind. "That's right," I told them both. "Just like the president."

This was the long-presaged moment I had been warned to prepare for. My son was born on the weekend that Barack Obama announced his candidacy. Since then, people have been telling me that his presidency would mean great things for my son. Indeed, this was one of Obama's privately stated aims. When his wife Michelle asked what he thought he could accomplish if he became president, he said: "The day I take the oath of office, the world will look at us differently. And millions of kids across this country will look at themselves differently. That alone is something."

True, it is something. But when Thomas is safely back in the station and the moment is over, it is not very much. Because for all the white noise emanating from the Tea Party movement, it has been black Americans who have suffered most since Obama took office. Over the last 14 months the gap between my son's life chances and his friend's have been widening. Unemployment, which has held steady in the rest of the country, is still rising among African Americans and stands at almost twice that of white people. For black teens, unemployment is 43.8%. Meanwhile, foreclosures among African Americans are increasing almost 50% faster than for whites. At this rate, my son will certainly look at himself differently after Obama's presidency - and not in a good way.

This could legitimately be the starting point for an indictment of Obama's presidency. Certainly if a Republican president were behind statistics like this, few liberals would be offering him or her the benefit of the doubt. But like most other criticisms of Obama, particularly regarding the economy, you would have to make the case that another viable contender could have produced better results in the same circumstances. He entered in a moment of freefall. Calling on him to provide a softer landing or a parachute is one thing. Demanding that he suspend the rules of gravity is another.

I think that case could be made, but it is not the argument I'm making here. The fact that the first black president is presiding over deepening racial disparities is just one of the more potent illustrations of how the relationship between identity and electoral representation has become untethered from broader social, political or economic advances and rendered purely symbolic. The corporate model of diversity, which seeks to look different and act the same, has firmly stamped its imprimatur on a kind of politics that owes more to Benetton ads than black advancement. Where we used to seek equal opportunities, we have now become satisfied with photo opportunities - a fact that satisfies some liberals, annoys most conservatives and does little, if anything, for the lives of those whose interests are ostensibly being championed.

"We have more black people in more visible and powerful positions," Angela Davis told me before Obama won the Democratic nomination. "But then we have far more black people who have been pushed down to the bottom of the ladder. When people call for diversity and link it to justice and equality, that's fine. But there's a model of diversity as the difference that makes no difference, the change that brings about no change."

This is not just true for race. India's upper house last week passed a bill to reserve a third of all legislative seats for women. Given that India ranks 99th in the world for female representation, this would make a significant difference to the Indian parliament if it becomes law. The prime minister, Manmohan Singh, described the vote as a "historic step forward toward emancipation of Indian womanhood".

Not necessarily. There is no absolute causal link between gender representation and gender equality. Six of the countries that rank in the top 20 for women's representation are also in the top 20 for per capita rapes. Meanwhile, a global gender gap index, compiled by the World Economic Forum, which assesses how countries distribute resources and opportunities between the sexes, reveals glaring discrepancies. Angola and Nepal, which stand 10th and 17th respectively in terms of representation, are 106th and 110th in terms of equality. Ireland and Sri Lanka, which rank eighth and 16th respectively for equality are 87th and 125th for representation. In 2008, two female party leaders locked horns in elections in Bangladesh, producing the second female prime minster for the country in a decade. According to the WEF, gender inequality in Bangladesh is bad (it is 94th) and getting relatively worse (in 2008 it was 90th).

This does not undermine the campaigns for more diverse political representation but should sharpen the arguments that support them. Representative democracies that exclude large sections of the population are not worthy of the adjective. Nor should the power of symbolism be underrated. Black Americans may have fared worst under Obama, but they are also the most likely to approve of his presidency. A Pew survey released in January showed the highest number of African Americans believing they are better off now than they were five years ago - even though economically they are not.

Moreover, in most cases difference does make a difference. While there may be no black or female experience, evidence suggests that a critical mass of certain groups can have an affect on outcomes. A 2008 study in the Columbia Law Review discovered: "When a white judge sits on a panel with at least one African-American judge, she becomes roughly 20 percentage points more likely to find" a voting rights violation. A 2005 Yale Law Journal study revealed not only that women judges were more likely to find for plaintiffs in sexual harassment cases than men, but that the presence of female judges increased the likelihood that men would find for the plaintiff too.

The fact that five of the 10 countries with the highest female representation are also in the top 10 for gender equality is no mere coincidence. Since the push for parliamentary parity is often part of a larger effort surrounding equal rights, greater representation is more likely to be the product of progressive social change than a precursor to it. The relationship between identity, representation and equality is neither inevitable nor irrelevant, but occasionally contradictory and always complex.

It's comforting to know there are simple words of racial reassurance I can tell my son when he's three. It would be even better to imagine that he would not be in need of that kind of reassurance by the time he reaches 23.