Vindicated at last! All of us misanthropic misery guts, whingers and whiners, Seroxat-refuseniks, "walking nimbus clouds"; we grouches, saddos, naysayers, demoralisers and party-poopers – our day has dawned. Time to gather and strike for the right to snigger, sulk and be sceptical, for the whole purpose of the cult of positive thinking is the beatification of bullshit.

To write Nickel and Dimed, about how America's working poor live, Barbara Ehrenreich took low-paid work herself. For Smile or Die, her latest instalment on what's eating America, having cancer was the personal starting-point for an investigation into the ubiquitous notion that positive thinking is essential to health, wealth and wellbeing. Positivity and magical thinking may actually make illnesses worse, prompt us to seek wars we can't win, make us waste time and money "improving" ourselves when the real impediments to happiness lie far beyond our control, and make bankers believe they're benevolent demigods.

Americans seem proud of being able to clap themselves into a frenzy of certainty for certainty's sake. "They had distributed motivational books and . . . they came to believe it themselves," writes Ehrenreich of the macho world of finance; "$3trn-worth of pension funds, retirement accounts, and life savings evaporated into the same ether that had absorbed all our positive thoughts." Optimism convinces you that cancer results from a deficient immune system and can be healed through meditation, or that Lehman Brothers would survive because Richard Fuld wanted it to survive. According to motivator Zig Ziglar, who helps companies such as AT&T bounce all the blame back on to the worker, if something goes wrong, it's because you didn't work hard enough or pray effectively. Boo! Boo!

Positive types aren't just misled, they're mean. "Negative people suck!" claims one American motivational coach, an exemplar of the "empathy deficit" in positive thinking. The pitiless message to the powerless from all these motivational speakers, megachurch preachers, self-help gurus and other assorted selfishness-sellers is that sad sacks get what they deserve.

Promoting the idea that happiness is within your grasp is in the interests of corporations trying to bamboozle an overworked and underpaid workforce. It's also favoured by churches trying to get rich quick off the American dream. Ehrenreich traces the fad from Calvinist self-control through Christian Science to blatant assumptions of the holiness of cash. Informing the uneducated and unmedicated that their plight is all their own fault is followed up by instructions for making anything you desire – from a new TV screen to a trip to Mexico – "materialise" through mind control. The censorship of negative opinion combines perfectly with the American policy of each man for himself in the best of all possible worlds.

This is the philosophy that gave us the smart bomb, the space programme, sub-prime mortgages, plenty of psychopaths and Sarah Palin. Every dumb American idea we've all had to stomach and die for can be attributed to this devotion to fantasy and self-satisfaction. Ehrenreich writes that America is unsurpassed in one area: "the reflexive capacity for dismissing disturbing news". Current American euphemisms for getting fired include "releases of resources", "career-change opportunities" and "growth experience".

It's when writing about the cancer industry that she's at her most eloquent. When she got breast cancer, Ehrenreich found that not only did she have to confront a life-threatening illness but also a whole bunch of idiotic pink products, from proud cancer-defying sweatshirts and breast cancer candles, to a teddy bear with a breast-cancer ribbon sewn on its chest.

Cancer victims are expected to exude happiness – otherwise you're apparently exposing yourself, and fellow cancer patients who come into contact with you, to toxic negativity. You might also make your friends uncomfortable. Ehrenreich was told by a Panglossian oncology nurse that chemotherapy smoothes the skin and helps you lose weight! But all the denial and courageous cookie-baking distract patients from questioning their treatment or why they got cancer in the first place. Ehrenreich attributes hers to taking HRT (a hugely lucrative industry); other factors may be diet and pollution.

Americans aren't happy, they're just trained to look as if they are. It's fake orgasm on a grand scale, and we're almost deafened by the din. Ehrenreich dares to mention the value of "defensive pessimism", that handy trait that suggests you keep your foot near the brake pedal just in case there's a three-year-old round the next corner. We want chefs who worry about the soufflé falling, we want energy planners who consider the worst outcomes of radiation poisoning and plutonium thefts, we want pushchair manufacturers to be wary of crushing babies' fingers. We need a grown-up disdain for complacency, compliance and conformity, and a critical forum in which you are not reviled for having nothing to advertise but your discontent.

Think about all the people over the years who've told you to embrace change, or think positive, or smile-love-it-may-never-happen. Were they right? I doubt it. I bet "it" did happen, and I bet you didn't like it.

Lucy Ellmann's Doctors and Nurses is published by Bloomsbury.