Priests,teachers and parents have for generations advised their wards io think twice before speaking,to count to ten when angry and to get a good night"s sleep before making big decisions.Social networks care little for seconcl thoughts.Services such as Facebook and Twitter are built to maximise"virality",making it irresistible to share,like and retweet things.They are getting better at it:fully half of the 40 most-retweeted tweets clate from January last year.Starting this month,however,users of WhatsApp,a messaging service owned by Facebook,will find it harder to spread content.They will no longer be able to forward messages to more than 20 0thers in one go,down from more than 100.The goal is not to prevent people from sharing information-only to get users to think about what they are passing on.It Js an idea other platforms should consider copying.Skeptics point out that WhatsApp can afford to hinder the spread of information on its platform because it does not rely on the sale of adverrisements to make money.Slowing down sharing would be more damaging to social networks such as Facebook and Twitter,which make money by keeping users on their sites and showing them ads.Their shareholders would surely refuse anything that lessens engagement.Sure enough,Facebook"s shares fell by 23%in after-hours trading,partly because Mark Zuckerberg,its boss,said that its priority would be to get users to interact more with each other,not to promote viral content.Yet the short-term pain caused by a decline in virality may be in the long-term interests of the social networks.Fake news and concerns about cligital addiction,among other things,have already damaged the reputations of tech platforms.Moves to slow sharing could lielp see off harsh action by regulators and lawmakers.They could also improve its service.Instagram,a photo-sharing social network also owned by Facebook,shows that you can be successful without resorting to virality.It offers no sharing options and does not allow links but boasts more than a billion monthly users.It has remained relatively free of misinformation.Facebook does not break out Instagram"s revenues,but it is thought to make money.The need to constrain virality is becoming ever more urgent.About half the world uses the internet today.The next 3.8bn users to go online will be poorer and less familiar with media.The examples of deceptions,misinformation and violence in India suggest that the capacity to manipulate people online is even greater when they first gain access to cligital communications.Small changes can have big effects:social networks have become expert at making their services compulsive by adjusting shades of blue and the size of buttons.They have the knowledge and the tools to maximise the sharing of information.That gives them the power to limit its virality,too. The advice of priests,teachers and parents is brought up to