Everybody secretly believes they're Sherlock Holmes.
Some are signed-up, cap-and-magnifying-glass-carriers. They lose the car keys, They spend 30 minutes looking for clues in the dust pattern on the kitchen bench.
Others are casual believers. People who watch couples in restaurants and think, "new haircut, uncomfortable shoes, pained expression from thong wedgie ... definitely, a first date".
Humans just like to think we understand other people's behaviour.
So it's no surprise that we've never lost interest in talking about age gaps in relationships. Inevitably, when we see a couple with a significant age gap, we reach for the deerstalker. We raise our eyebrows, give a little cynical snort, and say, "well, we know why they're together, don't we?"
Mona Dotcom was in it for the money, wasn't she? When Madonna dated Jesus Luz, it was for the name-based irony ... and probably the sex too.
This week, the world's favourite father figure, Stephen Fry, got engaged to his 27-year-old boyfriend, Elliot Spencer. And Twitter is ablaze; it's wringing its hands that Spencer is 30 years Fry's junior.
I've never understood the age gap problem. I'm currently nursing a crush on Marco Pierre White, 53. (Thank you, MasterChef Australia.) But, just from listening to the conversations, our society clearly has a problem.
From listening to my family, friends, and two very interesting ladies on the bus, I've been trying to piece together why our knee-jerk reaction is disapproval. I reckon it's because we question an older partner's motives in their relationship. When we see an older person with a younger partner, it strikes us as odd.
We want to know why they're together. If they were the same age, we'd think they were in love. But we seem to think love only exists between two similarly aged people. So we have to look around for other reasons.
The most common, and most acceptable, mixed-aged coupling is an older man with a younger woman. We assume she is his trophy. He is dating her for her body and the elastic pleasures of youth. She is in it because he's rich/powerful/can make important-sounding phone calls.
We make the same murky assumptions about Stephen Fry. There are murmurs that he's in it because his fiancé is a supple 27 years old. It's made worse because it's Stephen Fry. He's not supposed to care about pout and perky posteriors of youth. He's supposed to be the greatest gentleman of the modern era; intelligent, sensitive, and with a dashing taste in cardigans.
This means that people are unsettled by his apparent lapse into lust. They start to raise eyebrows, sip coffee pointedly, and start sentences beginning with, "Well ... "
But is it fair to be so sceptical?
Stephen Fry is highly intelligent. He's clearly a sensitive man who thinks a great deal and in depth about matters. If someone of his intelligence and compassion is dating someone, it's probably not because he just wants to root. He's probably going to want to talk to them too.
The same situation can apply with any normal older men. If they're smart and empathetic and kind, they'll probably want a partner who they can talk to. And if they're dating someone younger than them, isn't it possible they feel there's symmetry between their souls.
And yet, we still do the nudge, nudge, wink, wink - well he's in it for the booty, eh? Our cynical eyebrow-raising may make us feel superior. But at the same time, it discredits the idea that relationships can be formed on intelligence, friendship or humour.
Now that's getting dangerously close to saying men don't even value these.
We're assuming Stephen is just in it for the sex. We're repeating our favourite stereotype that men just want to hump everything.
We really are rubbish at putting intelligence, sensitivity and compassion into our idea of "being a man". We just assume the measure of a man is the measure of his dick.
Not only is this unfair, it's also wrong. What about all the male poets, activists or scientists? They're making it very clear they care about more than sex. So I don't think it's entirely reasonable to always assume every older man only wants one thing.
Don't get me wrong, I know there are lots of guys, and girls, like that. But surely not every man wakes up with the sole purpose of getting laid?
If anyone would ever be proof of that, it's Stephen Fry.
I reckon his writing has given so much to our understanding of humanity, that we at least owe him the benefit of the doubt.