We like to think of our personalities and qualities as relatively fixed -- your IQ on Monday is much the same as it is on Wednesday, and if you were an outgoing optimist last year, you're likely to be an outgoing optimist this year too. But science has repeatedly shown things to be massively more complicated. 

Not only do traits we tend to think of as 'fixed' actually shift dramatically across the years, but they can even swing wildly by the hour. Studies of standardized test scores have shown you're probably functionally way dumber in the afternoon than in the morning, and IQ can fall precipitously just because of a shift in mindset, for instance. 

Now new research out of Britain adds yet another wrinkle to the idea that your environment can have a significant impact on your personality and decision making. It turns out how risk tolerant you are varies significantly according to the day of the week, so that a bet that looks bold but smart on Monday might seem too terrifying to try come Thursday. 

Monday you is a bigger risk taker than Thursday you. 

Days and years are astronomical phenomena. Weeks are human inventions not tied to the movement of the earth and sun. The week is therefore just a convention, but it's a very old one. The idea of a seven-day week goes all the way back to the ancient Babylonians (though other cultures have tried different numbers since then), meaning we humans have lived by its rhythms for thousands of years. 

All those centuries of a seven-day cadence have left a noticeable mark on our psychology, according to recent research by the University of York's Rob Jenkins. Using data on hundreds of years of historical chess games, Jenkins and his collaborators discovered that players' appetite for risky moves slowly falls from Monday to Thursday before shooting up again on Fridays. 

Another study Jenkins conducted during Covid-19 confirmed that the more people's weekly routines were disrupted by lockdowns, the less this pattern of falling risk tolerance during the week held. There really does seem to be something about the usual rhythm of our weeks that impacts our willingness to take chances across the days. 

"The surprising implication is that the outcome of a decision can depend on the day of the week on which it is taken," Jenkins concludes, reporting his findings on the British Academy blog

He cites the Brexit vote, which happened on a Thursday, as an example. "The core message of the Brexit campaign - 'Take back control' - was a direct appeal to risk aversion, and the opinion poll data show that support for Brexit was strongest on Thursdays. Our analyses show that the outcomes might have been different had they been held on Fridays," he claims. 

Maybe don't decide on a Thursday then? 

That's interesting for politicians and election officials. But it should also interest another group concerned with the propensity to take on risks -- entrepreneurs. Starting a business always involves assessing risks versus rewards and making (hopefully) clear-eyed decisions about which bets are worth making. It's helpful to know that Monday you is likely to weigh risks a lot differently than Thursday you. 

Perhaps the best approach then is, whenever possible, to avoid making decisions which involve evaluating risks too quickly. Give it a week if you can and make the call only once you've had a chance to observe the whole spectrum of your risk tolerance as it shifts across the days. 

Or, if your heart is telling you to dive into a risky new venture, then aim to make the final call on a Monday. With a fresh week in front of you, you're more likely to summon the courage to actually pull the trigger.