Computer games are all about killing things. They are about how fast you can shoot. They are an evil stalking the land, stealing the minds of the young.

Well, that's the conventional wisdom, anyway. But Gallup's charts of the top 20 games for the PC show that half the most popular games are about building things up, not destroying them.

'We are seeing increasing sales of the type of game where the play is all about decisions and strategy,' says Stuart Dinsey, editor of CTW, the electronic games retailers' newspaper. 'Generally, PC owners today want something to get the grey matter going.' Seven million adults with a computer at home use it mainly for playing games, says the research company TGI.

Three weeks ago, a game based on the struggle to set up the first American colonies leapt into the No 1 spot. In the game, there are certainly battles to be fought - but the focus is on managing growth. In Colonisation, complex decision-making is needed to prosper. You can co-exist peacefully with native tribes and learn from them, or try to wipe them out; you can build up a trading empire based on raw tobacco, or invest in cigar production. You can also invest in training missionaries, and develop the skills of your settlers.

Colonisation was designed by Sid Meier, known for two other 'thinking' games: Civilisation, in which you invest in acquiring knowledge in order to see your civilisation advance; and Railroad Tycoon, in which you must plan and build up your railway network while other barons try to steal your market. These - and games like the popular Sim City, in which players have to manage the growth of a city - teach useful lessons: most obviously, decision-making about allocating scarce resources.

'The games market is much more cerebral than before,' says Roger Bennett, general secretary of the European Leisure Software Publishers' Association.

He claims that computer games can be of benefit to children, teaching important lessons and giving them an opportunity to experiment.

'I have a 15-year-old son who has learnt a lot from Premier Manager 2 (a game in which you take on the role of a football manager). It's simplistic, but an ideal way for him to start thinking about business and managing money.'

For Professor Stephen Heppell of the Ultralab at Anglia Polytechnic University, the key test of a game is: does it stretch their fingers or stretch their intellect? The Ultralab has been set up to find ways of making learning more 'delightful', and games-playing is an important part of its mission. For pre- school children through to pensioners, 'playing games develops new capabilities. How people harness and build on that in their school, home, university or office is a new challenge.'

Mr Meier says that in the United States children as young as eight have been using Civilisation in classes as a learning tool. In the core adult market, however, he faces entrenched prejudices. 'Many people, when they think of computer games, think of Space Invaders, Nintendo and stuff like that. They never get past that to look at games like Civilisation or Sim City. I think there are a lot of (grown-up) people who would like computer games if they could get past the barriers.'

Unfortunately, the barriers sometimes loom large. After two short weeks at No 1, Colonisation was knocked from its position by Doom II, an extremely violent and gory game from Virgin. Ah, well] Never mind the culture, pass the ammunition.