We tend to paint the past only in extremes, as having been either categorically better than the present or irredeemably bad. Maybe that’s why we live now in a culture in which many of us would prefer to break clean from what we perceive as the racist logic of previous eras — specifically the idea that the purity and value of whiteness can be tainted by even “one drop” of black blood. And yet, however offensive those one-drop policies may appear today, that offensiveness alone doesn’t strip the reasoning behind them of all descriptive truth.

In fleeing from this familiar way of thinking about race, we sidestep the reality that a new multiracial community could flourish and evolve at black America’s expense. Indeed, the cost of mixed-race blacks deciding to turn away could be huge.

With the number of Americans identifying as both black and white having more than doubled in the first decade of this century — from 785,000 to 1.8 million — such demographic shifts are bound to shape social policy decisions, playing a role in the setting and reassessing of national priorities at a time when Washington is overwhelmed with debt obligations and forced to weigh special interests and entitlement programs against each other.

Consider the impact that a broad redefinition of blackness might have on the nation’s public school system. In the past few years, the federal government has implemented new guidelines for counting race and ethnicity, which for the first time allow students to indicate if they are “two or more races.”

That shift is expected to change the way test scores are categorized, altering racial disparities and affecting funding for education programs. For this reason and others, the N.A.A.C.P. and some black members of Congress have expressed concern that African-Americans are at risk of being undercounted as blacks compete more than ever with other minorities and immigrants for limited resources and influence.

Scholars have long maintained that race is merely a social construct, not something fixed into our nature, yet this insight hasn’t made it any less of a factor in our lives. If we no longer participate in a society in which the presence of black blood renders a person black, then racial self-identification becomes a matter of individual will.

And where the will is involved, the question of ethics arises. At a moment when prominent, upwardly mobile African-Americans are experimenting with terms like “post-black,” and outwardly mobile ones peel off at the margins and disappear into the multiracial ether, what happens to that core of black people who cannot or do not want to do either?