Just how addictive are mobile games?

Candy Crush Saga has been downloaded more than 100million times on Android, and hasn't dropped out of the top ten games on the iTunes app store since January.

King, its publisher, has more than 66million players worldwide, and on any given day, 700million games will be played worldwide.

It's a free app with a very simple premise: match three or more of the same coloured 'candies' to earn points. But it is also earning its makers a daily fortune. For it was revealed by ThinkGaming this week that it makes £400,000 a day in revenue - £146m a year.

But how can a free app make so much money, and just who is spending so much money on the game?

The answer to the first question is in-game purchases, or what are known as a '"freemium" business model. Players can buy upgrades to help them through the game, and extra lives when they run out. Each paid upgrade costs between £0.69 and £27.99 - cheaper ones are used once over; more expensive ones last for the entire game.

Reports have centred around case studies of women aged between 25 and 55 (the group identified by King as the game's most loyal demographic) spending several hours at a time playing the game, at the expense of work, housework or even childcare.

[Casual gaming more popular among women]

Mark Griffiths, director of the International Gaming Research Unit at Nottingham Trent University and an expert in gambling and gaming psychology, explains why games like Candy Crush are so popular. 'Each game is potentially so short. That's been a staple of this kind of game since the days of Tetris. It gives you the ability to play for short periods of time - on your commute or in between tasks. It can be a massive stress relief.'

Successful game design has little to do with theme, graphics or sophistication. Griffiths explains that there are a few basic psychological factors at play here.

'One of the basic psychological principles of gaming, a building block of game design, is the idea of "operant conditioning". Like a slot machine, the game rewards some actions and punishes others, and we learn from this.

'If it's predictable, it becomes boring, so games use something called a random reinforcement ratio schedule. It keeps people responding to it for longer. Rewards come thick and fast, and their unpredictability leads to greater persistence in playing.'

The second major "trick" that keeps us glued to the games is the concentration factor.

'It's totally cognitively consuming - it requires 100 per cent of your concentration,' explains Griffiths. 'It doesn't mean you're addicted, but it is totally engaging.'

This certainly chimes in with tales of dinner burning on the stove or being late to collect the children from school that have been focussed on elsewhere, but Griffiths argues that the principle of total cognitive engagement is not necessarily a harmful one.

'It can be put to great medical use. Patients recovering from chemotherapy have been shown to need less painkillers if their minds are occupied by games - and the same has been shown to work on children with skin conditions.'

More worryingly, mobile "freemium" games make use of something psychologists call "suspension of judgement" - a concept seen more often in casinos.

'The big thing is to turn your money into a virtual representation of money. Casinos use chips; apps and websites use e-credits. It lowers the psychological value of money and makes it easier to spend,' says Griffiths.

The same principle is at work when we use slot machines, premium-rate phone lines and credit cards. 'If you don't think there's a difference, try paying with a fistful of £5 notes next time you spend £70 or £100 on clothes,' adds Griffiths. 'You will notice a different feeling, for sure.'

[My guilty secret: in-game buying]

But are they addictive? Should we be talking about addiction to games on the same level as gambling or other recognised addictions? Mobile games fulfil one important criteria for addiction: rewards can come quickly and games are short - the same reason slot machines are addictive, and playing the lottery twice a week is not. But there is a difference between time spent playing games and genuine addiction, believes Griffiths.

'Addiction has to be put in context. If it's having a negative impact on the rest of your life, it's addiction. But if you're someone who can spend hours playing a game without there being any negative consequences, it can be a life-enhancing experience. I've done studies with gamers playing for 14 hours a day; when one of their circumstances changed and he met his future wife, he played less. It might be unhealthy for other reasons, but it's not necessarily addictive.'

Jo Twist, CEO of the Association for UK Interactive Entertainment, added: 'There is currently no proven link between video games and addiction. We are however aware of some individuals that may play games excessively and that's why we actively promote safe and sensible gaming via our askaboutgames.com site, where we recommend that all gamers should take regular breaks of at least five minutes every 45-60 minutes.'

Addictive or not, mobile games are certainly proving a lucrative market for King and other game producers. Questions remain over whether more needs to be done to control in-app purchases in games such as Candy Crush.

'The games industry takes the health and well-being of all consumers very seriously,' said Jo Twist in a statement. 'It has a number of measures in place to ensure that games can be enjoyed safely and sensibly. We therefore recommend that consumers use the password locks and controls systems available on most mobile devices to control how games are played:these controls can be used to prevent access to in-app purchases, limit internet access and control access to age appropriate content.'

According to Mark Griffiths, there is a need for the games industry to go a step further in warning people about the levels of in-game purchasing required. 'People should have as much of an informed choice as possible. If you're going to be buying something in a potentially addictive structure, a financial warning would definitely be something I'd support.'