Whether she's discussing demographics, campus massacres or late-term abortions, the journalist and novelist Lionel Shriver always sounds pretty sure of herself. From time to time, though, regular readers of her Guardian column might notice some conflicts. Beneath the surface stridency, there's a tendency to pull off the odd rhetorical switcheroo. One moment Shriver is the über "anti-mom", hailing the pleasures of childlessness; the next she's urging British women to get breeding. One column finds her promoting career women; another claims true love is the only way.
Shriver's previous novel, the Orange Prize-winning We Need to Talk About Kevin, was acclaimed for its brilliant handling of its subject matter - the relations between a mother and her son, a high-school killer. Shriver's latest is less obviously violent, focusing on inner desires and turnarounds. Her heroine, Irina McGovern, is a fortysomething Russian American living in London. Life has been good to her and her American partner, Lawrence. Her work as a children's book illustrator and the ten years that she has spent homemaking bring her pleasure. Each evening, when he returns from his job as a researcher in a think-tank, she enjoys the "ordinary happiness - if there is such a thing" of togetherness. Their nights have their rituals. Carefully prepared meals with ingredients bought from Borough market; mutual sniping at the Late Review pundits; familiar late-night gropings. Irina regarded her relationship with Lawrence as a miracle:
She didn't care if feminists would have maintained that she didn't need a man . . . Shameful or not, having a man who loved her and whom she loved in return was the most important thing in Irina's life.
One night, Irina finds that the security of her world, and of this "most important thing", is shattered. The occasion seems innocent enough - dinner with a mutual friend, the well-known snooker player Ramsey Acton. Traditionally, Acton's birthday has been a foursome; this year, however, Ramsey is divorced and Lawrence is away on business. Irina finds herself feeling a little odd. He's a man almost utterly unlike Lawrence. A snake-hipped south Londoner, he smokes, drinks, knows almost nothing about the situation in Afghanistan, and notices Irina's ankles. Unexpectedly, somewhere between the sashimi and the teriyaki sauce, she finds herself wanting to kiss him.
At this point, the book splits in two. The form is similar to that of choose-your-own-ending children's books or the 1990s rom-com Sliding Doors, in which Gwyneth Paltrow gets to live life twice over, with completely different hairdos. In one version of the story, Irina kisses Ramsey. In the other, she crosses her legs and goes home. The chapters alternate between what happens in each version of her life - and the novel ricochets, much like Acton's balls, between an increasingly strained partnership with Lawrence and an arm-candy existence married to Ramsey, going ever-so-faintly snooker loopy.
Such a summary risks making The Post-Birthday World sound gimmicky - which it isn't. Reminiscent of both Carol Shields and Alison Lurie, the writing is continually engaging, the 1990s period detail rich, and the novel itself is a compelling take on the desire to have more than one opinion, or passion, at a time.
One might hazard a guess that part of its urgency comes from the fact that the dilemmas it presents are familiar to Shriver (who is, she admits, a keen recycler). In a recent interview, she spoke of her own change of partners. Her new husband makes her, she claims, "happier than any book". There is, however, a sense that this book is an attempt to work out some questions, both about residual moral qualms and about a surprising sense of her own conservatism, her new-found aspiration for women to "fall in love" above all else.
Part of this novel's delight is the way in which it is set up to operate like a controlled experiment. It allows us to see Irina experiencing what life would be like with both men, and - because the rest of her environment, and character, remains almost exactly the same - to perform a strange kind of "compare and contrast". But this is also part of its most interesting problem. If Shriver is, like her heroine, hunting for a moral text to illustrate, the novel leaves you at a loss. On the one hand, the consistency of Irina's character allows for a liberal feminist reading. It implies that, in the end, we are responsible for our own feelings and behaviour. Our choice of bedfellow will not shape us beyond recognition. And yet, the placing of so much emphasis on the "what if" seems tricky. In making Irina's romantic destiny seem the only interesting variable, Shriver appears to imply that meeting the love of your life might be contingent and accidental, but the consequences of missing such meetings are devastating. As Irina puts it, "one needs a man more than anything on earth". Or, at least, women need to think about men more than anything on earth. Of that much, at the moment, Shriver seems certain.