My raison d'être," says American psychotherapist Jo Ellen Gryzb, "is simply to make people a little less nice." It's been her mission ever since she found herself huddled in her bedroom with her husband one Christmas, whispering about how on earth they were going to get rid of their house guests. "I had no idea how to tell them they had overstayed," she says. "I was a complete walkover."

Gryzb returned to work at Impact Factory, a personal-development agency, and found herself in conversation with colleague Robin Chandler, who had similarly spent his holiday tiptoeing round friends and family. "I know what our problem is," declared Gryzb. "We're suffering from the nice factor."

The pair set about devising a workshop designed to harden us up and cut back on excessive manners; an etiquette class in reverse, if you like. It has been so successful that they are now bringing out a book entitled The Nice Factor: The Art of Saying No.

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Gryzb believes the symptoms of niceness are everywhere: every time we let someone off the hook, can't say no, avoid conflict to keep the peace, feel guilty when we ask for something, and get roped into something we don't want to do. She also believes our tendency to over-apologise reveals a worrying undercurrent. "If an apology isn't genuine," she says, "psychologically you are saying you're ashamed of yourself. It's almost as if you are apologising for your very being."

The number of people Gryzb and Chandler have come across in their workshops who make life-changing errors because they don't want to let others down is astounding. "One scarily prevalent issue is the number of people who get married but don't want to," says Gryzb. "They become caught up in the machinery – the dress, the reception, the cake, the ceremony – and don't know how to get out of it. We know of numerous women who spent the whole of their big day crying... and not out of happiness."

The problem, Gryzb says, is that we are brought up in a culture of pleasantries. The ethos of the saying that it's nice to be important, but more important to be nice, is ingrained in all of us. "There's a lot of subtle cultural messages," says Gryzb. "We get it right from birth when we are told good children don't cry. Parents brainwash us about what makes a good child and what doesn't. What's wrong with crying? What's wrong with saying you need something?"

Gryzb isn't the only one who thinks that niceness is holding us back. Next month sees the arrival of Asshole: How I got Rich and Happy by Not Giving a S*** About You, by New York author Martin Kihn. "I was the nicest guy in the world – and it was killing me," he says in the book. "My life was a dictionary without the word 'no'. If you asked me for a favour – even the kind of favour that required me to go so far out of my way that I needed a map, a translator and an oxygen tank – even if I didn't know you that well, I might hesitate a second, but I'd always say yes."

Kihn walked other people's dogs, traipsed out of his way to bring back the most complicated lunch orders for colleagues and handed over his money to whichever charity or sales scam asked for it. The result of such "kindness" was a dead-end job and a second-rate apartment.

While Gryzb recommends subtle personality changes, Kihn takes it a step further. He picked up tips from the masters – Donald Trump, Scarface and "the guy in my building with a tattoo on his face" – and decided to "blowtorch away my old personality and uncover the rock-hard warrior within". In his book, Kihn devises a "10-step programme to assholism" for anyone wanting to acquaint themselves with their darker side. He himself signed up to the National Rifle Association, started kickboxing, screamed at colleagues and ate garlic bagels on public transport.

What is refreshing about both Gryzb and Kihn is that they offer an antidote to simpering self-help books. This isn't about bettering yourself; it's about worsening yourself.

Gryzb's workshops are confrontational. The first thing she asks participants to do is turn to the person on their right and think nasty thoughts about them. And her advice if you do find yourself at the altar with someone you don't want to be with is stark: "Run away," she says. "Don't face up to it. Don't make a big announcement in front of all your family and friends, that would be awful. Just get a cab to the nearest airport."

She also recommends practising being less nice. Buy something you know you don't want just to exercise your right to take it back; tell someone on the phone you are too busy to talk to them when you're not. Small things maybe, but if well practised, that inner alpha personality will have much less trouble coming out when it is really needed.

Kihn's recommendations are a little more brutal. Show no interest in others, he advises. In the workplace, take credit for everything except mistakes, and conduct all conversations on speakerphone. "The speakerphone is a great way to avoid the inconvenience of having to listen," he writes. "This enabled me to start executing one of my original Asshole fantasies: to turn into one of those office jerks who are loathed even more than the person who steals yoghurt from the fridge."

Kihn claims his project led him on "a voyage of months which succeeded beyond my wildest dreams". And Gryzb has long sorted out her problem of overstaying house guests. "No longer am I afraid of being selfish or being seen as selfish," she says. "I don't offer apologies or excuses, nor do I put someone else's needs before mine. Becoming less nice has made me a nicer person."

'The Nice Factor: The Art of Saying No' by Jo Ellen Gryzb and Robin Chandler (Fusion, £10.99) is out on 20 March. 'Asshole: How I Got Rich and Happy by Not Giving a S*** About You' by Martin Kihn (Penguin, £7.99) is out on 3 April

Gryzb and Chandler's 10-step guide to getting tough

1. Don't smile

Smiling gives someone permission to think you don't really mean what you say as you have this big grin on your face

2. Stand your ground

Backing off is wishy-washy. Standing your ground gives weight to your intention. This means both physically and verbally

3. Tell the truth

Let the other person know what you are feeling. Let them know you aren't comfortable with what they are doing

4. Agree when it's unexpected

If someone tells you you're being silly, agree. They have no place to go after that. "You're a bit touchy." "You're right, I am." It takes the wind out of their sails

5. Don't point fingers

If you intend to tell someone you don't like their behaviour, start sentences with how you feel, not what's wrong with them. Pointing an accusatory finger will only make them more defensive

6. Make up a list of handy excuses

This will get you out of situations long enough to see clearly what's going on before you put yourself back in the fray

7. Change your mind whenever you want to

You have the right to change your mind, whether it's two minutes, two hours or two months after the fact. There will be times when you don't want to honour your commitments

8. Keep things short and sweet

Gabbling won't help, it just gives the other person rope to hang you with

9. Don't engage

Never apologise or explain. Don't supply fuel for someone to use against you

10. Get your 'no' in quickly

Set your marker right at the beginning of the discussion. You can always change your mind later, but if you say it fast, it's out on the table and can't be ignored