This was all rather useful for investigators, who didn't have to try and put together shredded documents or rely on recalled conversations – it was all there in email and internal message exchanges.

You (and the traders and their bosses) are probably thinking: Why were they so stupid?

It all comes down to basic biology.

(Read more: How to be a rogue trader)

Risky behavior like this might feel as though it has a peculiarly modern phenomenon, but it is one of the best examples of how humans are still rooted in the behavior of the jungle – and how young men are still governed by testosterone - according to Joe Herbert, professor of neuroscience at Cambridge University.

"Those JP Morgan traders were just doing what male baboons do," he said.

Young male primates demonstrate certain common characteristics such as aggression and risk-taking more often than females and older males.

"They are in competition essentially for mates, but in order to do that, they need food, position and a place in the hierarchy," Herbert, who spent time on City trading floors monitoring traders' brains as part of his research, argued.

"This is a good thing. If you're Lewis Hamilton setting off on a race and you don't take any risks, you'll always finish the race, but you'll never win."

(Read more: How to handle emotions while trading)

Another biological reason for heightened risk-taking and aggression is that in a primate society, young males are more expendable – a male baboon can inseminate more than one female, but a female can't be fertilized by more than one male at a time.

The practices of the primate society are still evident in the 21st century. Young men have immature frontal lobes in their brains until their mid-twenties. They also have more testosterone, handed down from our primeval ancestors, which encourages risk-taking.

Despite the inroads made by women in recent years – including Ina Drew, chief investment officer at JP Morgan Chase while the Whale trades were being made - trading floors are still male-dominated.

"The primeval function of testosterone is to get the male to reproduce, but to do that it has to do other things," Herbert pointed out.

(Read more: Why a hungry trader is a risky trader)

On top of being guided by prehistory, the indiscreet traders are also being fooled into a false sense of security by modern technology.

"People feel this sense of anonymity and immunity to being overheard when online," Michael Price, senior lecturer in Psychology at Brunel University, told CNBC.

"It's also part of the culture where everybody knows what's going on and approves it. It's part of risk-taking in the culture to not care about who is looking over your shoulder."

The close bonding and camaraderie between young males can add to this behavior, according to Herbert.

"In most cases, this risk-taking is of advantage to them and to their group. If they don't (have controls), then you get a Lehman Brothers where the system collapses," he said.

This is why it is important to have some form of societal control on the young males by older males and females, he added.

"The banking crisis is a good example of where this control has broken down."

—By CNBC's Catherine Boyle. Twitter @cboylecnbc