How have contemporary artists impersonated people from different backgrounds as the basis for art? What have their performances revealed about the fears and fantasies that are projected on to racial and sexual "others"? Focusing on work by four US-based female artists active in the past 40 years, Cherise Smith highlights the powerful urge to merge. The daughter of mixed-race parents, whose frizzy-haired white mother was once described as "that black girl who tries to pass for white", she departs from personal experience to relate key artistic projects to the civil rights, black power, multicultural and "post-identity" eras from which they emerged.

Perhaps the most intriguing of the artists she discusses is Adrian Piper. One of the few black women to play a key role in 1960s conceptual art, Piper exploited racial and gender prejudices in adopting the male guise of the "Mythic Being". Sporting an afro and sunglasses and puffing on a cigar, this aggressive young black man "embodied everything you most hate and fear", as one of Piper's projects proclaimed. He cruised white women, staged a (fake) mugging of a white man, and took out enigmatic Village Voice ads where his picture was accompanied by inappropriate extracts from Piper's (real) teenage diaries.

In contrast to Piper's antagonistic stance, Eleanor Antin, as the black ballerina "Eleanora Antinova", adopted stereotypically female behaviour. Glamorously dressed, with hair frizzed and skin darkened, she recounted years with Serge Diaghilev over copious quantities of sherry. When she failed to get the attention she desired, Antin/Antinova lashed out: in her 1983 book Being Antinova, she accuses a waiter who ignored her of racism, and speculates that a cab driver who praised her tan thought she was going to rob him. Smith unpicks these extreme pronouncements, seeing them as a reflection both of Antin's identification with oppressed minorities and her problematic eroticisation of blackness, and relates Antin's art to the history of "blackface", in which immigrant performers tried either to camouflage their ethnic origins or to accentuate them.

Less insightful, however, is Smith's discussion of orientalism. To say that Antin "labours" on behalf of Edward Said's theories accords her art a secondary status - and, moreover, gives an oddly reductive role to Said's landmark work, which brilliantly synthesised but hardly invented critiques of the West's fascination with Eastern promise.

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Empathy replaces seduction in Anna Deavere Smith's Twilight: Los Angeles, 1992, a theatre and film work in which the African-American playwright and actor performs as 25 different characters based on people she interviewed in the wake of Rodney King's beating at the hands of Los Angeles police. Where critics have applauded Twilight for getting under the skin of individuals from different backgrounds, Cherise Smith views its virtuosic imitations sceptically, arguing that it reinforces, rather than destabilises, stereotypes.

Also working in a pseudo-anthropological manner, the Korean-born artist Nikki S. Lee immerses herself in various subcultures, including skate-punks and rednecks, hip-hop clubbers and Wall Street yuppies. For her 1997-2001 Projects photographs, Lee adopts the style, body language and attitudes of members of those groups. But Smith regards the undertaking as only skin-deep, and criticises it for promoting a depoliticised, post-identity model of affiliation.

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I welcome Smith's willingness to grapple with the ambivalent feelings these artworks provoke. But by seeing art in strictly ideological terms, she often misses the playfulness, perversity and absurdity of these idiosyncratic projects. Ultimately she fails to account for mimicry's transformative potential - how we unconsciously start to resemble and ventriloquise people close to us, how we stage our everyday personae, and how we both lose and find ourselves when we imitate those different from us.