That's life. Or is it?

A series of surprising studies into the science of the mind has shown how deeply our own beliefs influence the outcome of experiments

- Ben Ambridge

'Want to lose weight? Buy smaller plates." "Mindfulness at work: a superpower to boost productivity." "Leaving Facebook can make you happier." That's what the headlines and Ted Talks would have you believe. But are any of these psychological tricks - or life hacks, as they are often called - actually true?

The truth is, we don't know; and, in a sense, we can't ever know, because of limitations inherent in the design of the relevant experiments - in just about all experiments in what we might call "lifestyle science". That, at least, is the implication of a new study by a pair of Stanford psychologists, Nicholas Coles and Michael Frank. We'll get to their work shortly, but first I'd like to take you to the German city of Mannheim in 1988.

It was here that psychologist Fritz Strack conducted a study that has since become a staple of psychology textbooks. Experiment participants were given a cover story: that previous research using questionnaires had excluded participants who were unable to use their hands to fill in the form, and that this study would explore the feasibility of instead holding the pen in your mouth. Half the participants were asked to hold the pen in their teeth (which forced their mouth into a smile) and half in their lips (which forced their mouth into a neutral pout) while they viewed a selection of cartoon strips. The participants who were smiling when they saw the cartoons rated themselves participants as more amused than the who were pulling a neutral (if slightly odd) expression.

When they were asked afterwards whether they'd suspected anything fishy was going on, no participants showed any sign of realising that the cover story was simply a way to get them to smile. Strack seemed to have shown that at least sometimes - our facial expressions determine our moods, rather than vice versa.