Doctors have suggested it could be the sign of a personality disorder, sociopathy or narcissism, psychologists that it might be why people never get a second date - is being rude to serving staff is a deal breaker, a red flag and a window to the kind of person you really are?

You should never trust anyone who is rude to waiting staff or shop assistants.

Everything you need to know about a person can be quickly distilled from watching the way they interact with people in the service industry: I'm not sure what's worse, outright unpleasantness or treating someone as if they're invisible.

At least the former means they're noting the physical presence of the person, the latter suggests they deem them so insignificant that they're not even worth acknowledging and that to do so would be a bit like asking the vacuum cleaner if it had any summer holidays planned.

New research shows that saying thank you and being grateful isn't just basic manners and entry-level decency, it's also good for our physical and mental health: psychologists Emmons and Stern have published papers which reveal the hidden benefits of not behaving like self-important, ignorant, pig-headed, graceless gits when out and about.

'Gratitude has one of the strongest links to mental health and satisfaction with life of any personality trait—more so than even optimism, hope, or compassion,' they write, 'grateful people experience higher levels of positive emotions such as joy, enthusiasm, love, happiness, and optimism.

'They can cope more effectively with everyday stress, show increased resilience in the face of trauma-induced stress, recover more quickly from illness, and enjoy more robust physical health.'

Of course if you're the kind of person who needs to derive some kind of personal gain - other than the joy of spreading goodwill to all man, child, teen and womankind, obviously - from not being an obnoxious boor, then the battle is already lost.

You should be nice to people serving your dinner or scanning your shopping regardless of whether it'll put hairs on your chest or lower your health insurance premium: you should be nice because not doing so makes you a really awful person like Donald Trump or Katie Hopkins.

I've been a waitress (people clicking their fingers at me like I operated on a motion sensor, people shouting 'oi!' across crowded rooms at me to make yet more demands), I've been a shop assistant (people throwing money at me, perfectly fit and healthy people watching in silence as I packed their heavy shopping and then transferred it to the collect-by-car service at the end of a long Christmas night shift) and I've been a cleaner (a client admitted to me when I left that she'd often hidden money around her house 'to test you').

I have nothing but respect for anyone in the service industry who work their backsides off in jobs where I watch them regularly get short shrift from the public.

Thanks to recessions and credit crunches and increasingly difficult conditions for many businesses, there's a horrible smugness creeping in amongst a certain breed of customer, a hatefully self-important sense of 'you need me more than I need you' which encourages Little Emperors and Empresses to think they can behave like imperious toddlers before a much-needed nap with the added bonus of a 'the customer is always right' get-out-of-gaol card which absolves their behaviour entirely.

I have become one of those people who attempts to strike up conversation with shop assistants and waiting staff (I back off if they're not interested, I promise) which in short means that I have become my mother, in this small way, at least.

It doesn't take a great deal of personal hardship to ask someone how their day is going or say please and thank you, does it? No need to open the medal drawer for the most basic level of civility.

But...I have lost count of people who have told me that I'm the first person who has properly spoken to them at work that day or who are obviously delighted that someone wants to engage with them: this says far less about me than it does about the way so many customers treat the people serving them.