Britain seems to be suffering a growing problem with gambling addiction. John Phillips / Empics Entertainment A survey in 2010 suggested that nearly 460,000 people in the UK were "problem gamblers". That's defined as someone who scores three out of 10 or more on a psychological test for "pathological gambling".Since then, the number of people gambling in this country has gone up, from about 35 million to about 42 million. A spokesperson for the gambling charity GamCare told BuzzFeed News that problem gambling is likely to have gone up by the same proportion. That would mean there are around 560,000 problem gamblers in the country. Part of the reason appears to be the rise in "fixed-odds betting terminals", or FOBTs. Katie Collins / PA ARCHIVE IMAGES They're gambling machines, like the slot machines that you see in pubs, except that FOBTs have higher stakes and payouts. They've been legal in betting shops since 1999; gamblers can bet up to £100 a time, and win up to £500. In the decade and a half that they've existed, they've become near-ubiquitous: There are nearly 35,000 machines in the UK, and they took £1.6 billion, after winnings, from punters in 2014/15, according to a parliamentary briefing paper. It says that it is not clear whether they are behind the gambling-addiction surge, but that many people believe they are part of the problem.GamCare says that more than a quarter of people calling their gambling addiction helpline are using FOBTs. If you include slot machines, that figure goes up to nearly half. Addiction researchers say that FOBTs are particularly addictive because of the way they're designed. John Phillips / Empics Entertainment The machines are "at the pinnacle of addictive behaviour", says Prof Marcus Munafo, a psychologist at Bristol University who specialises in addiction research. That's because they offer frequent, but irregular, rewards. "What's key is that it's slightly irregular," says Munafo. "The nature of FOBTs is that winning feels somewhat random, but you win often enough that you feel there's a chance."He points to animal studies that show similar responses. "If you're an animal pressing a lever and you get food every time you press it, and then it stops giving you food, you learn quickly to stop," he says. "But if there's a random element – if it usually doesn't give you food, but sometimes it does – then when it stops giving you food, it takes you longer to stop." The compulsion – the addiction – is much more robust. The FOBTs are disproportionately deployed in poorer areas. Phil Cole / PA ARCHIVE IMAGES "They're not distributed at random in the high street," says Munafo. "They're concentrated in the most deprived areas."That's unlikely to be a coincidence, he says. Addiction is "very socially patterned". While smoking levels among the more affluent, for instance, have plunged dramatically in the last few decades, "they've hardly changed in the last 50 years among the least affluent".That may be to do with a lack of other options for the poorest. "People say [smoking] is their only pleasure in life," says Munafo, "because the rest of life is tough. Smoking is a relatively easy and quick way of having some thing to look forward to, even if it's expensive." FOBTs, and gambling generally, may fulfil a similar role.Another factor, he says, may be that a payout of hundreds of pounds "is more important for low-income people than high-income". That increases the sense of reward, and may make gambling more addictive. And gambling addiction in general is particularly prevalent among poorer people. John Stillwell / PA WIRE According to the British Gambling Prevalence Survey, "high-time, high-spend gamblers" – the ones who spend the most time and money on betting – "displayed the most adverse socio-economic profile [and] were more likely to live in areas of greatest deprivation, live in low income households and be unemployed". Gambling addiction appears to run along the same lines as drug addiction – as a quirk of our brain's reward system and its control system. Christopher Furlong / Getty Images "There's more similarities than differences [between drug addiction and gambling addiction]," says Dr Andrew Jones, a psychologist with Liverpool University's addiction research group. "There are similar underlying behaviourial traits to both. Addiction is mainly an imbalance between the reward system and our control system."Our brains reward us with a feeling of pleasure when we eat, or have sex, and other things that, in our evolutionary history, helped us survive and reproduce. "We have a reward system for good reasons," says Munafo. "It evolved to force us to pay attention to where we found food, to learn from our experiences."That's obviously a necessary system," he says. "Where it goes wrong is where that is hijacked by non-natural systems. The classic example is cocaine." Cocaine stimulates the reward system directly.The other factor is our impulse control system – that is, our ability to stop ourselves from doing things that feel good right now, to defer reward. The classic example is if someone is asked whether they want £5 now or £50 in six months' time. "People do vary in reward deferral," says Munafo, "and it seems to be associated with addictive behaviour." People have started trying to treat gambling addiction with drugs that block the reward system. Christopher Furlong / Getty Images The Times reports that the National Problem Gambling Clinic (NPGC) in London has started a trial prescribing naltrexone to problem gamblers. Naltrexone has previously been used to treat drug and alcohol addiction, by blocking receptors in the brain which make drinking and drugs feel good. The NHS trust which runs the NPGC said in a statement that so far they have treated five people with the drug since November 2015, in a pilot trial "designed to establish clinical effectiveness and demand in a UK population of problem gamblers". The drug costs £68 for a three-month course."It works better than nothing [for drug addiction], but not terribly well," says Munafo. "Addiction is a tough nut to crack." Jones agrees that the evidence for drug treatment for drug addiction "isn't terrible. It's OK." He says that it's not as clear that the drug works for gambling addiction, but since the same systems are involved, "if the drugs are targeting the same things, there's no reason why it wouldn't work".