Big investors have called on Apple to develop software that limits how long children can use its smartphones.The call came from two investment groups that hold $2bn (£1.48bn) of Apple stock between them.A letter calling for the digital locks, signed by Jana Partners and a California teachers’ pension fund, was sent to the iPhone maker this weekend.The call for better controls was welcomed by academics studying youngsters’ use of technology.Design conflictJana Partners and the California State Teachers’ Retirement System (CalSTRS) called on Apple to consider the impact excessive use of smartphones had on the mental health of young people. The two are worried that if Apple does not address growing concerns about smartphone use, its stock market value and general reputation could be damaged.According to a Reuters report, half of US teenagers believe they are addicted to their mobile phones and feel the need to respond immediately to messages. Sonia Livingstone, professor of social psychology at the London School of Economics, said it was good to hear the call from the investors. She added there needed to be one voice between device manufacturers, social media companies and internet service providers (ISPs) on the issue of smartphone use. “For a long time the concern has been to not do anything that would impact a friction-free experience,” Prof Livingstone told the BBC. “Everyone would like to have a well balanced life, but the way that devices are designed currently causes a lot of conflict with parents.”She called on Apple and other device manufacturers to have all notifications on smartphones switched off by default and for the creation of occasional reminders that urged youngsters to take a break from their phone after long periods of use. Prof Livingstone, who also runs a parenting blog, did question the use of the term “addiction” for those who spend a long time using a smartphone, however. “Everyone will agree that there is excessive use and even obsession with smartphones, but I don’t believe it’s addiction,” she said. Apple has not yet responded to requests for comment.