The 12 surprising signs your partner is cheating on you, by the Infidelity Coach teaching husbands EXACTLY how to have an affair - and get away with it!

About 50 per cent of people are unfaithful, according to scholarly estimates, and I'm gazing into the eyes of a serial offender.

Albert Arnaiz claims to have had more than 100 affairs while in serious relationships, and is now on a mission to tell you, or indeed your spouse, how to get away with doing the same.

Welcome to the morally uncomfortable, but undeniably intriguing, world of the Infidelity Coach. For £85 an hour, Arnaiz helps people successfully conduct an affair, perhaps by assisting them in creating an online dating profile or covering their tracks after the event.

Now he has written a book with the entirely self-explanatory title How To Be Unfaithful Without Getting Caught. Eighty per cent of his coaching clients are men, he tells me — but the book is proving surprisingly popular with women, too.

Is he a monster? Barcelona-based Arnaiz is blithe, yet businesslike, about infidelity, which understandably fills some with contempt. He has a boyish charm (which is no mitigation) and unlikely as it sounds, no malice.

He's 43, snooker-ball bald and his day job — unexpected, this — is a college lecturer in marketing in his home city. I suspect his opinions would feel even more scandalous if his English were fluent. Has he heard from many people upset by what he does? 'Just a couple of angry messages,' he says.

Naturally, one of his rules in conducting an affair successfully is to avoid detection. He warns against bringing a lover back to the marital home, for example, not because the lying and betrayal would be the height of disrespect, but because 'it's very... the word is dangerous — you get caught!' he says, grinning wolfishly. 

His book is relentlessly explicit (you've been warned), with thorough direction on deceiving your beloved.

My plan, however, is to persuade him to lay bare his cheater's charter on behalf of anyone suspecting their spouse of being unfaithful. Arnaiz knows this and is happy to help. Prior to our Zoom call, he promises 'fun'.

There is an art to disloyalty, he says and he's perfected it by means of 'a lot of experience'. He embarked on his first affair 'because I was in a long-term relationship with a partner, and it wasn't good. I tried to break it off — but we continued... and I decided to escape, and my way of escape was infidelity'.

How light and easy he makes it sound. This relationship lasted 13 years. It's the longest he's had. Perhaps, unsurprisingly, he's never been married.

I'm disturbed and fascinated. Is he a tin man, without a heart? He shifts uncomfortably and mentions 'very personal problems', which makes me suspect there was far more distress and pain in that first episode than he's prepared to admit.

So, why do cheaters cheat?

'There are a lot of people who feel a lack of attention, a lack of love, lack of passion, lack of sex. So they need to find these sensations, these emotions, these situations they have lost in their relationship,' he says. He thinks there are 'danger zones' at the end of every decade (29, 39, 49), with midlife a particular flashpoint.

'My clients are around 40 because they have energy, money and time. And this combination and this age, between 40 and 50, I think it's the best moment to have an affair.'

At 59, you're also likely to have the time and money to play away, but 'there is no energy, if you know what I mean', says Arnaiz. (I'm not so sure. I have friends who would not agree.)

Arnaiz insists the majority of men aren't looking for impossibly glamorous women to cheat with. For 20 per cent, yes, 'it's about looks', he says, but for most 'it's about the person. It's about the feelings, the connection, the way of thinking, the humour'.

Then he says something quite unexpected: 'It's more emotional than physical for men.' Clearly this is the opposite of what we expect of affairs — and no comfort at all to women who've been cheated on. 

Arnaiz notes in his book that for many women, their man's 'emotional infidelity' is more hurtful than his illicit sex. Physical attraction must be present, he adds, but much of it, unromantically, is also about convenience.

'A lot of people tell me his lover works with him, or her.' He mentions a study that found around 30 per cent of affairs happen at work. Loneliness is a common cause. Or 'always the routine, with kids, with family, with friends, and people think, 'Is that all life has for me? I want more!'. 

Marriage itself can precipitate trouble, with some people thinking, 'Wow, I'm in a kind of jail'. Sometimes we need to escape. Just one night in a month, or just once in my life.'

He reckons if there were no consequences, up to 80 per cent of men and women would play away.

Ironically, Arnaiz is in a committed relationship himself, and they have a six-year-old child. Less ironically, they met on an infidelity website — an online forum specifically for individuals who are married or partnered and looking for a secret affair.

Rather surprisingly, considering his moral attitude, they stopped the affair before ending their main relationships, and then waited a while before getting back together. Why?

'It's necessary when you break a relationship to be calm, passionless, to think about what happens to you, to your environment, to friends, family.' He adds: 'Maybe you're asking, 'Do I trust her?' ' Surely the question is: does she trust him?

He declares, 'A hundred per cent trust, it's never guaranteed. She could be unfaithful to me. And I could be unfaithful. That's life. And we have to handle it.' He bursts into hearty laughter, before adding, 'And it's not an open relationship'.

In general, Arnaiz does not recommend getting serious about an affair. He advises ending it 'after three months if you meet more than weekly; and after six months, if you meet three times a month'. More than this and you'll end up getting caught.

I don't really buy this bloodless, transactional attitude towards betrayal, however. In the past, for my work as a journalist, I've sat in on a therapy group for people who've been cheated on, and their emotional devastation has stayed with me. One compared it to a bomb being detonated in his life.

Arnaiz shows no such compassion. He believes the cheated-upon are in some ways to blame for their own deception.

'In an infidelity case, there are always three people involved,' he says. He prefers 'responsibility' to 'fault', but says it's shared three ways 'more or less'.

But what does the betrayed spouse do to make them responsible? 'Not seeing, not understanding, not communicating,' says Arnaiz. It's important to start talking, see what's happening and start to find solutions'.

In another life, Arnaiz could have been an excellent relationship therapist. Still, I'm dubious that, as he maintains, cheating can make you a better person and partner. He says: 'You could become a better version of yourself because it allows you to know yourself better.' 

You might even realise that you're 'making a mistake . . . and so come back to the marriage'. (Then again, he says, you might also realise 'I need to start a new life, maybe single, maybe with a new partner who makes me really happy'.)

Let's say a cheating partner wants to come back to their marriage. Why shouldn't their spouse boot them out? 'I know when you realise someone is unfaithful, it hurts,' says Arnaiz. 'But it's not the end of the world.' His advice is, don't divorce in haste. 'Think very clearly, with a cold mind, about what you want in life.'

Arnaiz says you should ask yourself: 'Do you think you could forgive this person or not? If you believe that people can make mistakes and you could allow them a second chance, do it.'

He says that in writing a book to help cheaters 'start an infidelity and do it right [ie, not get caught], believe me or not, I don't want to hurt the other part of the relationship — the partner, the wife, the husband'.

Even if you doubt that, the fact is: 'The book could help people who are very suspicious about their husband or wife and want to know if they might be being unfaithful. A lot of people have told me, 'It works for the person being unfaithful, but it works for the partner too.' '

Testing that theory, here are a dozen of Arnaiz's top signs that your spouse may be up to no good (see below).

How To Be Unfaithful Without Getting Caught by Albert Arnaiz is available on Amazon.

He's suddenly doing the dishes... and other clues he's playing away 

You see small changes in his digital behaviour

Arnaiz's advice on how a cheater can cover their digital tracks is comprehensive and pain staking. A watchful spouse should check if he's always facing the door while on his phone or laptop when you walk into the room.

This is to hide what's on a screen. Or perhaps he's turns off his data-spewing smartwatch at odd times, so it doesn't record 20 minutes of aerobic activity at 11am when he claims to be visiting his mother.

He's deactivated his Google Maps history

Realistically, says Arnaiz, most men become careless with tech or slip up. People use multiple apps but 'don't know all the functions and features of each'.

For example, with Google Maps, 'most people don't know there is a history, and you can see where your husband has been in the last three months on a map'. If he has deactivated it — why? And if he hasn't, how does he react when you ask to take a look?

The car is spotless and reeks of air freshener

The car is one place Arnaiz recommends for illicit sex, although not too often, and not the family vehicle. 'If you share your car, it's a place that your partner knows well, so any change of smell, a stray hair, could be terrible,' he advises. 'If he cleans the car once a month, and now does it once a week, that's strange. If he suddenly starts buying air fresheners, that's also odd.'

You find new clothes in a rucksack

Arnaiz warns cheaters not to invest in a new wardrobe, because it looks suspicious. But he accepts that most people who play away can't resist the vanity of buying some nice outfits, so suggests that, if they must, they keep their new threads together in a bag or rucksack. So check his bags!

He suddenly becomes a gym-goer

Whether the motive is to facilitate their absences, or get in shape for their new object of lust, habits often change during the course of an affair.

Mistakes include abruptly going from slothful to super-fit. 'They want to look good for their lover so, overnight, they start exercising.'

You notice he's often getting cash out

Arnaiz advises clients to pay for anything affair-related in cash, but not to get it out in advance or to hide it. 'Get the cash when you think you're going to use it.' Some, however, will neglect this advice. A wallet left on a table with a large amount of cash inside is a possible red flag.

He's started doing more housework

Occasionally, a chore-averse man turns into Mrs Mop. It's guilt, says Arnaiz. 'He doesn't feel good about himself and wants to be more helpful in the house.' This is contrary to Arnaiz's advice, of course, which is 'you have to act normal, that's the key'.

Unfortunately for the cheater, a 'biological consequence' of an affair is that his mind is often elsewhere, so he may be distracted in conversation, too. So watch out for daydreaming.

He's tired and says he has insomnia

Being unfaithful is 'mentally exhausting', says Arnaiz. 'The brain wastes energy thinking about two parallel lives, and it's necessary to sleep better, or take naps, do a little bit of exercise, or take supplements to keep your mind and your energy up.' Consequently, he suggests the cheater should claim insomnia to cover for their tiredness.

Some men might also become bafflingly chatty, says Arnaiz, 'talking more, about his work, his hobbies, his friends — anything to avoid thinking about what he's doing'.

You're fighting less and have more sex

Arnaiz says: 'I think more sex makes you want more sex. It's normal. And there's less fighting because he is happier.'

He stops wearing his usual cologne

Arnaiz advises his clients not to wear cologne or perfume when they meet their lover. 'Smell is very sensitive,' he says. 'If you both wear a perfume or cologne when you kiss, hug or have sex, your scents mingle It's easy for your wife or partner to detect that strange smell on you.'

He's never available at lunchtime

Arnaiz believes lunchtime is the best time for a lovers' tryst. Even if you work at an office and have just 60 minutes, 'you can eat in ten minutes and the rest of the time...'

He's seeing friends more often

Arnaiz teaches his clients to tell their spouse, 'I'm meeting friends next Friday', and then go and actually meet them, but arrive late or leave early to make time for the affair.

'If you have a routine with your friends or hobbies, take advantage of that to be unfaithful,' he says. So pay closer attention to his timings!