We think of this as a new thing.

Selfie only made it into the Oxford English Dictionary in 2013, when it quickly became word of the year.

But selfies are about as old as photography.

The first was taken in 1839 by an American named Robert Cornelius.

So why have we used photography, this miraculous invention, to take pictures of something we can see in our bathroom mirror every morning?

Something odd is clearly going on.

And who better to explain human oddities than Sigmund Freud?

Freud invented psychoanalysis and popularised many ideas like the ego, the unconscious, and talking to a therapist.

One of those ideas is narcissism, or excessive self-love.

In a Greek myth, a young man called Narcissus sees his reflection in a pool and spends so long staring at his own beauty that he loses touch with the rest of the world.

And eventually drowns trying to embrace his own image in the water.

Freud thought that a bit of self-love was a natural part of being human.

But Freud also thought that self-love can turn into a psychological disorder, when someone loves himself to the exclusion of everyone and everything else.

And that's what we usually mean by narcissism.

Psychologists have developed tests for measuring personality traits like narcissism.

Here are some results.

Narcissists do tend to be more active on social media.

And posting selfies is strongly related to narcissism, but only if you're a man.

Women tend to be less narcissistic than men, even though women post more selfies.

Perhaps more worryingly, narcissism is rising.

The psychologist Jean Twenge has shown that, over the past few decades, it's risen at roughly the same rate as obesity.

Freud derived most of his insights from everyday life observations, so he would have been very interested in all this data.

He would have concluded that narcissism is only part of what's going on in the rise of selfie culture.

Some people are posting selfies not because they're in love with themselves, but because they want other people to be in love with them.

Freud would have seen that need for approval as neurotic or hysterical.

Freud began his career in the late 1800s, a much more sexually repressed time.

Men and women were kept strictly separated.

And they were taught to be ashamed of feeling... sexy.

Many of Freud's female patients in Viennese high society suffered from 'hysterical paralysis' - an inability to walk that had no physical cause.

Freud believed that these women were, without knowing it, stopping themselves from walking because they wanted attention.

So, if we need attention so badly that we'll paralyse ourselves for it, why not post a few selfies? Isn't that better?

Well, Freud would find something unhealthy about selfies – not just because of what they say about the people taking them, but also because of what they do to the people seeing them.

Selfies show people's best moments, carefully curated and heavily stage-managed.

So we're increasingly surrounded by images of other people's apparently perfect lives and bodies.

Recent studies show that this makes us feel more envy, inadequacy, isolation and insecurity.

Making us, in Freud's terms, more neurotic.

Freud said, "the aim of psychoanalysis is to replace neurotic misery with ordinary human unhappiness".

So next time you reach for the camera, remember Narcissus and focus on your friends instead.

You may not get as many likes, but you'll get a thumbs up from Freud.