There's a song in the musical Chicago in which the characters bemoan the state of the modern world. "Whatever happened to fair dealing, and pure ethics, and nice manners?" they sing. "Whatever happened to 'Please may I...' and 'Yes, thank you' and 'How charming'?"
These days, it's a question many others on stage are asking - of their audience. Recently, the theatre producer Richard Jordan complained about sitting through a production of Doctor Faustus starring Kit Harington (of Game of Thrones) which was more like feeding time at the zoo.
"The couple to my left ate their way through a large tub of popcorn during Act I, while the couple on my right chomped through a packet of crisps," wrote Jordan. "It was like listening to eating in Dolby Stereo." Worse, a couple in the royal circle returned after the interval with a large box of Chicken McNuggets and a side of fries.
It wasn't just food. Theatre-goers also commented loudly on the action - and so many used their phones to take pictures or videos that the ushers gave up trying to stop them.
But is it just audiences who, as Chicago would put it, ain't got no class? Or is it a symptom of a deeper problem?
There seems little doubt that, in London's West End at least, behaviour is getting worse. The influx of TV and movie stars, and their fanbases, has resulted in a trampling on the unwritten rules of conduct - an ongoing, night-by-night culture war between Their Sort and Our Sort.
Dominic Cavendish, the Telegraph's theatre critic, spent the press night of Guys and Dolls (starring Hollywood comedienne Rebel Wilson) having his chair kicked by the wriggling, guzzling teenagers behind him. (Sample comment: "Oh look, there seems to be some place down there where there's a band.")
But the phenomenon, he says, is more widespread. In ultra-respectable Hampstead, the elderly couple in front of him recently spent much of the performance putting on an inebriated show of their own, before the wife eventually slumped on to the shoulder of the long-suffering critic of the Daily Mail (who says that, given the quality of the play, he "rather sympathised"). Another source told me of a scandalous recent occasion when a Russian patron at the Royal Opera House not only left their mobile phone on - but actually answered it. And clapping between movements is now actively encouraged at the Proms. What would Sir Henry Wood think?
Yet it's not just the theatre and concert hall. Ladies' Day at Aintree has become a cavalcade of poor taste, with scantily clad attendees quaffing champagne through "booze funnels". Buses and trains are filled with discarded fast-food bags - and with the thudding beat from headphones. Even MPs now clap and cheer a departing prime minister, rather than murmuring and waving their order papers.
So what's going on? Why has Britain, the nation of the stiff upper lip lost its sense of decorum?
The most obvious explanation is that we are living in a less formal age. In an educational film from 1953, children were taught to answer the phone as follows: "Hello, this is the Merenti residence, Benny Merenti speaking." Now, the model is Homer Simpson's simple: "Ye-llo?" If your bank calls, they will ask for "Joe", not "Mr Bloggs". We are less deferential, less uptight, less mannered in every sense. Even the Queen has flattened her vowels.
By itself, this is no bad thing. But alongside a lack of deference is a lack of restraint. We want what we want - and resent anyone who attempts to deny it to us, or insists that we must follow inconvenient rules. Think of Lewis Hamilton, baffled and infuriated at being turned away from the Royal Box at Wimbledon for not conforming to the dress code. Or of the theatre audiences who, as Cavendish says, see a play not as a cultural experience, but as a chance to pamper themselves, complete with copious snacks and plastic cups of champagne.
This goes hand-in-hand with the way that, increasingly, we all think of ourselves as stars in the movies of our lives - lives which are not so much experienced as captured and displayed on the internet.
One of the most interesting social changes in recent decades has been a staggering rise in narcissism: between the 50s and 80s, the proportion of American teenagers agreeing with the statement "I am an important person" went from 12 per cent to 80 per cent. Alongside that, studies show a collapse in empathy: we think more of ourselves and less of others.
That trend is fostered and focused by social media. Today, it's more important that the meal looks good on Instagram than that it tastes nice, more important to prove you were at the gig - or the play - by whipping out your camera than to actually experience it. All this is reinforced by how compelling our phones are, how easy it is to keep gazing on them and let the rest of the world fall away. We've all seen people staring at their phones on the street, oblivious to those they bump into: many of us have been that person. The new mobile phone sensation Pokemon Go reminds its players to stay aware of their surroundings every time they commence a session - but judging by the behaviour on the streets, it's pretty much a lost cause.
For some, there is no way back from this situation. "People used to get on in the world by being considerate to others," says Mary Killen, the Spectator's etiquette expert. But now, they just don't get the practice. "When and where are they going to interact in groups and see that belching, gobbling, and talking in loud voices are habits that need to be curbed?"
Cavendish, for his part, describes the way that Harington's fans "stood like zombies outside the stage door night after night, waiting for selfies as he emerged. There was hardly any chatter, nothing like human interaction. That I think marks a break from the past - whereas historically audiences could be vocal and rowdy, people are now just content-grabbing, or seeing the event on stage as a sideshow to their digital lives."
It's true that we all have experiences that make us fret about the future of civilisation. There was the neighbour in the cinema who alternated between checking his iPhone and covering me with droplets of carelessly expelled mucus, or the time I found myself behind the then director-general of the BBC at the opera and realised he was checking the England football score every five minutes.
Yet there are also seeds of hope. Those at the highest end of the cultural spectrum say standards are standing firm - to the point, says Gillian Moore, director of music at the Southbank Centre, that performers from overseas will often mention how impressed they are by the audience's focus and concentration. The bigger problem, she insists, is with traditionalists taking it upon themselves to maintain unrealistic standards: her 19-year-old son was recently told off by a neighbour for nodding his head during his first awed experience of Wagner's Ring cycle.
Even as the barbarians advance, then, some citadels are holding out. And there is renewed hope of those at the top of society setting a good example. "Give us a kiss!" yelled the photographers as our new prime minister stood with her husband Philip on the step of No.10. Sensibly, Mrs May refused.
By the book: debrett's decrees for modern living
We all know that moderation is the mother of good sense, and overindulgence is socially unattractive, but complete abstinence can sometimes seem anti-social and holier-than-thou.
Eating in public
Not only rude (and often hypnotically revolting) but it can play merry hell with our digestion. Keep noise to a minimum and never talk with your mouth full.
Switch off your phone, or turn it on to vibrate, when you are going into meetings, theatres, cinemas and so on. People in the flesh deserve more attention than a gadget, so turn off your phone in social situations, and don't glance at it longingly mid-conversation.
Normal teenagers are just differently mannered: swallow your distaste. Hail their occasional appearance during daytime hours with good grace, welcome every grunt and use humour to chip away at the wall between you and them.
Think carefully before hitting "Send" if the message is written in haste or when emotions are running high. Your message may be stored permanently, and there is no such thing as confidentiality in cyberspace.
Robert Colvile is the author of The Great Acceleration: How the World is Getting Faster, Faster (Bloomsbury)