Increased regulation of US technology companies is one of the biggest risks to a sector that has led stocks higher, potentially derailing the market's near-decade-long bull run if it suffers a sustained sell-off. The S&P 500 tech sector has risen 5 per cent in price terms this year, outpacing the S&P 500 which has gained 1 per cent. Speaking at the Reuters Global Investment 2019 Outlook Summit in New York, Pacific Investment Management Co Global Economic Advisor Joachim Fels said potential increased regulation of social media companies remains one of the major downside risks to higher equities.