How do people justify earning more than others?

There's been much discussion over WPP boss Martin Sorrell earning £63m in a year. But how do you decide if one person deserves to earn more than someone else? Workers earning more than £27,000 - the UK median salary - explain why they think they're worth it.

"If I have a bad day I could do serious harm or kill a patient. That level of responsibility has to be reflected in the pay," says Zoe Norris.

She earns £64,000 a year as a locum GP and says she reached her current position by "20 years of hard work".

Norris, 35, says doctors cannot "claim poverty" - but argues their pay should be "commensurate" with the responsibility, years of training and ongoing study.

She says their income is at the "lower end" compared with people of similar levels of education, adding: "I earn less than an MP earns and I work more than an MP does."

In a 2011 YouGov survey, the amount of "responsibility" a worker had was chosen as the most important factor in how much people "ought to earn".

How well a worker did the job came second in the poll, which asked respondents to choose up to three answers.

But public opinion on what people deserve may not be reflected in labour market reality. The UK median salary is £27,000 per annum.

John Purcell, of City headhunters Purcell and Co, says people with "rare" skills will always earn more than those who are "easily replaceable".

Purcell, whose firm specialises in banking and finance workers, says: "You have to consider how intelligent, hard-working and qualified these people are. Not many people can do what they do."

So what reasons do people give when asked why they deserve more than others?

Gail Reynolds made £192,000 in 2015-16 selling Avon beauty products and from commission from 350 sales people she has recruited (and more people they recruited).

She is uncomfortable talking about what she "deserves".

But Reynolds, who works with her husband Brian, says: "We deserve it because we continue to change people's lives."

She says: "I meet girls who are 21 to 25, they are single, they are on the dole, they don't think they can do anything. If I can talk one person away from thinking that they can't do what I can do, I have earned my money."

Reynolds says "anyone" can do what she has done - but she had the "entrepreneurial spirit to make it work".

"Maybe there's something inside of me that makes me a little bit different; that makes me worth the money I earn," she says.

Another worker - an HR director at a FTSE 100 company, who asked not to be named - says she finds her salary of almost £150,000, potentially rising to more than £200,000 with bonuses, "a bit surreal".

She accepts it's a lot, but she says it reflects the "accountability and responsibility" of her job.

She says every person "around the executive table" works "all the time" - and she has never met someone at that level "who doesn't live for work".

She adds: "You have to be quite a strong person to still have a family life."

Many workers make versions of the same argument - that their job involves suffering, and this should be reflected in their pay.

Finn Brennan, who drove London Underground trains for 23 years and now works for union Aslef, says drivers deserve their £49,673 annual pay because they do a "difficult and skilled job".

He says they start work as early as 04:45 and finish as late as 01:30, working eight-hour shifts in a "small metal box underground" - conditions he says would be considered "cruel and unusual punishment" in other circumstances.

Whenever a Tube strike comes around, it's not hard to find voices arguing that drivers are paid too much. Brennan says Tube drivers' pay has been protected by a "strong and well-organised trade union", adding: "We are not overpaid. The reason it looks good is that so many people in London are grossly underpaid."

But do workers overestimate their "worth"?

In 2014 TSB chief executive Paul Pester defended his £877,500 salary. He accepted it was "a lot of money" but cited "the responsibility that comes with" running a bank with 4.5 million customers.

Luke Hildyard, of the High Pay Centre, a think tank which argues the gap between high and low earners is too wide, says the idea that successful firms thrive due to "great individuals" at the top gives executives "too much credit".

No-one interviewed for this article said they were overpaid - although Colin Ross, a "happiness engineer" for app firm Buffer (which publishes its pay scales online), says he feels "incredibly lucky" with his £87,000 salary.

Ross says he is "not a superstar" in terms of technical skills but he is experienced and knows a lot about the code behind Buffer's web application.

"I like to think that I'm approachable and insightful enough that other people feel able to reach out to me," he adds. "To me, the most valuable employees are those that combine skills, experience, flexibility, insight and approachability."

One issue may be that people's expectations rise faster than their pay.

Research by salary comparison website Emolument suggests workers with more academic qualifications and more experience - both likely to correlate with higher pay - are less likely to be satisfied with any bonus they receive.

The figures also showed that workers in financial services - not noted for low pay - were the most dissatisfied of the industries studied.

Emolument says bankers are told not to discuss their pay - "often a sackable offence" - and this leads to a "high level of paranoia". Separate data from 5,000 London bankers showed 73% were "dissatisfied or unsure" about their latest bonus, with more senior staff least likely to be satisfied.

So as pay goes up, workers may feel less and less like they're getting what they deserve.

Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox