Being likeable could be costing you tens of thousands of dollars

There was a time when five of us were were hired at a large corporation at the same time - two men and three women, all with similar experience and similar levels in the organisation.

One night at after-work drinks we started talking about our salaries (we were a close team) and it turned out I was on $30,000 more than my counterparts.

My female colleagues were aghast and enquired further, with a certain level of hurtful injustice in their tone. I just shrugged and said that I simply asked for it in the final interview. One of my female colleagues exclaimed that she could never be that forthright - but I didn't feel that I was being rude, I was simply negotiating.

I was less agreeable. I was polite enough to be likeable, but not polite enough simply to accept whatever was first offered. My female colleagues were so focused on being likeable, they forgot to be a little bit selfish. And it turns out they may not be alone in struggling to negotiate.

It can be inconvenient in these times to suggest that there are differences at a population level between men and women, and hard to articulate to people who are more driven by ideology than by science and data.

The reality is that there are significant differences between men and women that aren't simply sociological constructions - there are biological, empirical, measurable differences.

The key is about understanding those differences, and then using that knowledge to assess how we might do things better.

One of the big psychological differences between the sexes is the aforementioned personality trait, agreeableness. Along with openness, conscientiousness, extraversion and neuroticism, agreeableness makes up what is known in psychological literature as the big five personality traits.

While the big five are not that great at predicting individual things, like what choice you as an individual will make when selecting a movie on Netflix, they are very good when looking at trends at a population level and attributing a "more likely to do X" statement based on how strong you are in each of the five traits.

When looking at gender differences, it turns out that at a population level, men tend to be significantly lower in agreeableness than women.

At first glance, this may seem to be a win for women. People high in agreeableness tend to be more patient, kind, modest, cheerful, loyal and polite. All extremely useful traits when getting along with people and forming strong social bonds, but not so great when negotiating.

People lower in agreeableness are often more impolite, less fearful of being offensive and more comfortable with not being liked if it gets the job done and they end up with a better deal. In a dog-eat-dog business world, lower agreeableness in males, to some degree, means that overall men are more likely to negotiate better conditions, and could even explain why they are over-represented in the world of start-ups and entrepreneurialism.

The right mix of agreeableness seems to be somewhere in the middle. Agreeable enough so that people like you, but not so agreeable that you are worried a selfish request might cause offence.

When people ask me about progressing their career and appropriate salary expectations, I often say: "The value of your work is very much like a bottle of fine wine or an amazing piece of art - it is worth what someone is willing to pay."

The problem is, you only know how much someone is willing to pay if you learn how to negotiate - and high levels of agreeableness don't always work in your favour at the negotiating table.

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