This post is adapted from my Indy Voices article of the same title originally published on 24/08/23

The new splurge now, justify the cost later craze has taken social media by storm. Good, harmless fun? Not by my calculations, says mathematician Kit Yates

f you want to get a lifestyle craze trending on social media, then you could do worse than stick the word “girl” in its name.

That’s how we got “hot girl walk” – a trend promoting both the mental and physical health benefits of walking – and “lazy girl job” – a phenomenon extolling the virtues of improving your work life balance. There’s also “girl dinner”, which sent the internet into a frenzy over its simple prescription of low-effort snack-based dining, and now the newest addition to the crowd: “girl math”.

Many of the “girl” trends have had their share of criticism, from accusations that lazy girl job promotes low career aspirations to suggestions that girl dinner might encourage unhealthy dietary choices.

But, to me, “girl math” feels a bit more troubling than the previous trends. It originates from a New Zealand radio show in which women ring in to describe and justify expensive purchases they’ve made. The hosts then use some basic maths to help the caller feel better about the money they’ve blown. For example, to justify buying a $1,000 designer bag, “girl mathematicians” might give it a five-year lifetime and calculate its cost per day of just 55 cents. Add in multiple uses for the bag (eg handbag, cabin bag, shopping bag, festival bag) and you might find it’s actually saving you from having to spend more money on other items.

It’s a funny trope, which is why “girl math” videos are getting millions of views on social media – particularly on TikTok. It’s also getting people to do a little bit of real-world maths which (although not super high-level) from my viewpoint as a professional mathematician is always welcome.

Indeed, calculating the daily cost of a purchase over its lifetime is not a terrible suggestion when it comes to budgeting, although it’s a calculation you should really do before making the big-ticket purchase rather than to justify it afterwards.

I have a couple of gripes with the trend, however. My first issue is with the name itself. Mathematicians have fought and indeed continue to fight a battle for representation in our traditionally male-dominated subject. We constantly struggle against tropes that suggest maths isn’t for girls or that “girls don’t like hard maths” – a theory advanced by the government’s former social mobility tsar Katharine Birbalsingh.

So I find the idea that there is maths for girls (the “girl math” trope typically employing fairly straightforward calculations) and by implication maths for boys, hard to stomach. As I have argued previously, it’s important that we continue to reinforce the idea that all of maths is for everyone.

Sometimes the logic underlying girl math TikToKs can leave a little to be desired. For example, under the unwritten rules of “girl math”, returning an item of clothing that costs $50 and then buying another item with a price tag of $100 means that that second item, in fact, only cost $50. Again, I know it’s a joke, but the justification of clearly flawed reasoning by its association with girls, as if this is a trait specific to women, doesn’t do anything to dispel long-held and damaging stereotypes.

Similar to the “women are bad drivers” trope (which is a pernicious myth), this sort of stereotype can seep into the view that the caricatured group hold of themselves. “Dubious math” would be a far more accurate name for the trope, as well as removing its stigmatising impact, but of course, it wouldn’t capitalise on the “girl” trend and perhaps would never have gone viral in the first place.

My other issue with the trend is that, for some people, “girl math” may become more than just a joke.  Some of the more worrying tropes that “girl math” relies on are that “cash isn’t real money” or that purchases under $5 are “pretty much free”.

Indeed, in line with the “cash isn’t real money” trope, there is evidence to suggest that people pay in cash for purchases they find harder to justify to themselves so that there is no electronic paper trail. This makes it easier for the guilt associated with the offending purchase to be forgotten and similar purchases repeated in the future. Reversing a seductive “girl math” calculation for the $5 trope, it’s also worth working out that a $5 purchase every day of the year adds up to $1,825 over the course of a year – a significant portion of most people’s annual budget.

I’m all in favour of spending our hard-earned salaries on the things we enjoy and which make our lives better – we need these perks to lighten our moods in these difficult economic times – but it’s important to remember there is a literal price tag attached to those purchases.

I am advocating for spending our money with agency and intention – budgeting and planning in order to make the treats we buy ourselves viable in the long term, avoiding the post hoc “girl math” justifications that could lead to unsustainable spending habits.