Games like Candy Crush start off free, but users are soon paying for extras
Simple rules and a colourful appearance have earned digital games such as Candy Crush fans of all ages worldwide.
But while many of these games are free to download, they can soon start to cost money.
Many games now prompt users to use real cash to buy more "lives" or chances to win levels. Prices for these packages can range from 99c to as much as €75.
Candy Crush is one of the most popular online games worldwide and is available to download on smartphones and tablets.
Social games such as this can get young users hooked, experts warned.
John O'Connor, who lectures in Visual Arts in DIT, said the addictive nature of some online games would cause "concern for educators and academics in gaming".
Mr O'Connor said: "There's no doubt these games are designed to tap into our psychological desires and needs.
"They are designed to feed that instant gratification - that little prize or little win. That gives the player a little bit of satisfaction.
"Children, like adults, can get sucked into the games they play."
According to King, the company that manufactures Candy Crush, the gender split among game players is 50/50.
It added that its games were solely marketed at adults.
It said players who did pay for extra chances tended to spend about $25 (€22) per month.
It makes contact with the most active users to make sure they are comfortable with their levels of gameplay and payment.