The Women's prize-winning Nigerian author talks about sexism, hairstyles and living in America

Chimamanda Ngozi Adichie's third novel, Americanah is the story of two Nigerian émigrés who love and lose each other across continents and years. It is a book about leaving, and loneliness and the intersection between class and race, all of which makes it sound rather hard work – unjustly so. It is a book about hair: straight versus afro; and discreet tensions, not just between white Americans and Nigerian immigrants, but between Africans and African Americans, between the light- and dark-skinned, between new and established immigrants, and its frankness – in particular on the subject of gender – has upset some people. "I knew that was coming," says Adichie. "I can't write a book like that and then go, 'Oh my God, they're upset.' But my intention wasn't to upset." She smiles. "It's just that I'm willing to if that's what it takes to write the book."

We are in New York, which Adichie is visiting from her part-time home in Maryland, and where the following evening, Americanah (the word refers to Nigerians returning home after living in America) will win best novel at the National Book Critics Circle awards, beating the favourite, Donna Tartt's The Goldfinch. At 36, she has three much-acclaimed novels behind her – her first, Purple Hibiscus (2003) was longlisted for the Man Booker prize and shortlisted for the Orange, which she won in 2007 with her second novel Half of a Yellow Sun, and Americanah is just out in paperback – and a film version of Half of a Yellow Sun out next month. Adichie is in a position to do precisely as she pleases – such as divide her time between the US and Lagos, where she owns a home. In fact, if her husband didn't work in Baltimore, she says, she would probably "live in Lagos all of the time and just come to the US when I want to go shopping." She hoots with laughter.

Fashion is a big part of Americanah. Ifemelu, the heroine, comes to the US at 19 and is faced with a choice: in this new country, what version of herself will she be? She adopts and then sheds a fake American accent. She burns her scalp with hair relaxant to "Europeanise" her hair and then reverts to an afro. (The novel opens with a scene in a hair salon outside Princeton where she sits for six hours while the African hairdresser listens, incredulous, to her decision to go home.) She starts writing a blog called Raceteenth or Various Observations About American Blacks (Those Formerly Known as Negroes) by a Non‑American Black, cataloguing the judgments and sympathetic failures that this description entails, including the fact that in America, she observes, everyone is constantly being diagnosed with mental illnesses that don't exist on her continent. No one, Ifemelu muses, has a "panic attack" in Kinshasa. "Did things begin to exist only when they were named?"

These aspects of the novel describe Adichie's own experience of leaving Nigeria at 19 to attend college in the US – although Ifemelu stays away for 13 years without a trip home, while for Adichie it was "only" four. She is shrewd on the subject of white American self-confidence, the ease and largesse of deep-seated privilege. There is the husband of the woman she babysits for, "brimming with his awareness of his own charm". There is the man she dates, a white American called Curt who is achingly sensitive and clocks many of the racist slights she is subject to, meanwhile displaying "an infantile quality … that she found admirable and repulsive". When Ifemelu dates Blaine, an African-American professor at Yale, she herself is found wanting. "She was not sufficiently furious," writes Adichie, "because she was African, not African American." It is only when Ifemelu comes to America that it occurs to her to think about race at all. "I only became black when I came to America."

It is a peculiarly Nigerian response, Adichie says, one rooted in her country's history and in cultural attitudes, which, she laughs, have made Nigerians hated and resented across Africa. She grew up in a middle-class household, the daughter of academics and the fifth of six children. While her siblings all followed her parents' wishes and became "very responsible" – "the doctor, the pharmacist, the engineer" – Adichie asked to drop out of medical school and, if she passed the scholarship exam, go to America to study "communications". Her parents were baffled but gave in. "They thought I was slightly strange, but my parents are quite lovely; they're progressive. Still, it is a standing joke in the family that I say to them: 'The only reason you supported me is because my sister was already a doctor.' If not, I think they would have said, nobody leaves medical school, and especially not for reasons such as 'I'm not happy.'"

Compared to the strict educational environment in which Adichie grew up, the US education system seemed extremely slack. In Americanah, Ifemelu marvels at how students open their mouths without having anything much to say; how everyone gets an "A" and can take tests more than once. How they are "all flush with knowledge, not of the subject of the classes, but of how to be in the classes".

By comparison, Adichie says, she felt she had no appropriate schtick. "I remember my first year in undergrad I was sitting in class and just looking around, utterly confused. I thought, what are they saying? It was kind of a performance. And I felt so inadequate because I didn't know how. And I think at some point I learned to play the game, but it's just not me. It's very hard for me to bullshit."

Given the way men and women behave in meetings, one imagines her inhibition was a gender thing, too.

"Yes. When women do say something, not only is it more likely to be ignored, but the women themselves are accepting of it being ignored. So they're less likely to push back. That happens less often in Nigeria. Women in corporate settings are more likely to be vocal."

Why does she think that is? "Because although there's a lot of gender bullshit in Nigeria, I think women in the west have a lot more invested in being liked. And being liked if you're female means a certain thing. So in workplaces, women who are bosses in Nigeria are fierce. The people who work for them, men and women, respect them. But, these are women who very keenly perform gender stereotypes when they go back home. And if they give a public interview, they have to say, 'My husband supported me and allowed me to …'."