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For Emma Lovell, one of the biggest downsides of her old job is muscle strain. And it is not so much from standing - although she had to be on her feet all day - but smiling.

"Your face literally hurts all the time after a while," says the 25-year-old, who has worked on and off in promotion for the past six years. "Some days I find it hard to smile; it's just not realistic."

While handing out flyers or offering free food at festivals may seem like reasonable work, the actual hard slog, says Lovell, is the emotional KPI that clients expect her to fulfil.

"Happiness is a major part of the brief. The number-one thing is always 'be fun, excited and energetic'. And on a quiet day or when customers are rude, putting on a happy face can be taxing."

Practically every day, we engage in the buying and selling of feelings. From the smiling barista who remembers our names to the taxi driver who listens to tales of our crappy day – we not only pay for the services they provide, but also the emotional extras that occasionally accompany them.

Sociologist Arlie Hochschild calls this manufactured warmth "emotional labour" – a term defined as “the management of feelings to create a publicly observable facial and bodily display”.

This can include anything from feigning happiness and suppressing anger or frustration to conjuring gratitude when you don't necessarily feel it.

On the surface at least, it may sound deceptively like the definition of being "civil". But the question of exactly how ethical it is to pay someone to be happy at work – especially low-wage, customer-facing work – has recently re-emerged following reports on the strict (and unintentionally comical) behavioural codes of the UK-based fast food chain Pret a Manger.

Timothy Noah of New Republic noted that staff at the popular sandwich chain are required to follow a detailed set of conducts that the company calls “Pret behaviours”.

The requirements listed more than 50 behaviours employees must master, including such things as “has presence”, “creates a sense of fun” and “is genuinely friendly”. What's more, they don't want anyone who “minces words”, “agrees blandly with others” or is “just here for the money”.

To ensure staff are kept in a state of “enforced rapture” at all times, the company also hires mystery shoppers to visit every outlet once a week. If the service happens to be unsatisfactory, then the whole team misses out on a bonus – a significant punishment given that it represents a potential 16 per cent lift from their base minimum wage.

The bottom line, writes Paul Myercough at the London Review of Books, is that Pret workers aren't supposed to be unhappy: “They are recruited precisely for their 'personality', in the sense that a talent show host might use the word.”

In fact, this growing trend of “enforced happiness” isn't unique to the fast-food industry. For example, according to a leaked 2007 training manual,l, Apple Store staff are forbidden to use the word “unfortunately” – due to its “negative connotations”. Instead, they are asked to use phrases like “as it turns out”. They must also follow a set of behavioural codes that are presented in the brand-happy format of an "APPLE" acronym.

Last year, RMIT also launched a controversial 12-page “Behavioural Capability Framework” for its academic staff, requesting professors and lecturers to show a commitment to “passion” and “enthusiasm”, and display a “positive rather than negative” attitude at work.

University of Queensland social psychologist Fiona Kate Barlow believes emotional labour – no matter what industry we're in – is becoming a growing part of our work.

“We typically think of labour as being either something very active, like building a house, or something very cerebral, like writing reports or crunching numbers ... But emotional labour is probably going to come into 95 per cent of all jobs.”

So what happens to our psychological health when we find ourselves smiling in the face of insults or repressing anger from an unreasonable boss or customer?

“The feeling of constantly having to feign happiness when you feel none or little, or having to mask your true authentic self can be very problematic,” adds Dr Barlow.

“In the West, in particular, when we suppress emotions – anger, disappointment, frustrations - or even our positive emotions, it can lead to a range of associated mental costs, including stress and depression.”

“To some extent, [our culture also has] very strong norms around 'being yourself', about speaking up and speaking out. And when we feel that we can't do that, we do feel extremely upset.”

It's worth noting that jobs that involve heavy emotional labour are typically very “female-centric” – owing in part to the gendered nature of our workforce. “When we talk about the lower-paid, lower-status jobs, women are disproportionately represented. So it's very likely that women are going to be doing emotional labour more than men in jobs,” says Dr Barlow.

According to latest ABS figures, there are also more female workers in care-giving and service-oriented professions – with 79 per cent women in healthcare and social work, 70 per cent in education, and 55 per cent in retail.

Jeannine Paterson, a midwife at the maternity unit at Nambour General Hospital for Queensland Health, thinks that since empathy is considered natural for women, their emotional labour can often go unnoticed – and therefore not adequately compensated for in the workforce.

“The male midwife at our hospital is very much admired by the women that come through our hospital," she says. "[On the other hand], it seems that the female staff are expected to be caring and don't receive the same accolades as a male in the same role.

“[It's often assumed that] for a man to be in this caring role they must be something special, so they seem to receive far more recognition than any of the women who not only perform the same job but then have to go home and care for everyone else.”