The Magic Of Thinking Big is one of those marvellous 50s self-help books that's a pleasure to read, so long as you don't make the error of actually doing what it says. David Schwartz's bestseller is a time capsule from postwar America, in which ambitious auto salesmen give themselves pep talks in the shaving mirror and filing clerks dream of being boss. ("Eliminate the word 'impossible' from your speaking and writing vocabulary," Schwartz counsels, as if that might be doable, or helpful.) Today, Schwartz's idea of "thinking big" looks small, conventional and focused on commerce: he wants you to be all you can be, but what he assumes you'll want to be is the leading rental apartments agent in the Chicago metro area.

Many of us like to think our ideas of success are more sophisticated: we understand happiness isn't all about money, work or being the best. We prize "balance" or, if we're relentless workers, tell ourselves we do it for fulfilment, not cash. But, according to Harvard Business School professors Laura Nash and Howard Stevenson, we're kidding ourselves if we think this will make us happier. Our new definitions of success, they write in their book Just Enough, can be more problematic than the old ones.

It's easy, for example, to mock prescriptions for happiness based on ever greater wealth: we know that people who make £1m just end up wanting £10m. But the focus on cash is only half the problem. The other is the infinitude - and consequent unachievability - of the goal. "Am I making the most of my life?" Nash and Stevenson point out, is just as infinite a question as, "Am I rich enough?" albeit more spiritual-sounding. Set it as a target, and you'll never get there. (This casts in a different light those exhortations to "live an extraordinary life": when do you know you've achieved it? Also - a troubling question - why are we so scared of being normal?)

The obvious objection is that such goals aren't meant to be completed, but to propel us ever forwards. Yet in Nash and Stevenson's interview-based studies, the truly happy weren't locked in constant striving after a single goal, even that of "making the most of your life". Nor, though, were they living in "work-life balance". That, the authors write, is another unattainable target. Our multiple goals will naturally clash, and seeking a static, permanent, clash-free state is a recipe for misery. (Which doesn't mean, of course, that policies making it easier for people to combine work and parenting aren't admirable - just that hoping they'll bring perfect "balance" is futile.)

For Nash and Stevenson's happy people, by contrast, the secret of success was that there was no single secret. They had multiple goals, accepted that they were incommensurable, and moved sequentially between them, achieving plenty, but expecting no moment when everything would snap into place and they could say they'd done it.

In this, they actually were Thinking Big, although David Schwartz would probably have called them unfocused.

• oliver.burkeman@theguardian.com