Few investment tools have enjoyed as much recent success as exchange-traded funds. Since the launch of the first mainstream ETF a decade ago, they have attracted $1.2 trillion, according to BlackRock. Last year alone, 600 new funds were launched, compared to just 77 falling by the wayside.

But pension funds have yet to make up their minds about ETFs. Some hold billion-dollar positions, such as the $100bn Texas Teachers Retirement Fund, which has $3.35bn in the Vanguard MSCI Emerging Market ETF. Others, such as the UK's £9.6bn Strathclyde pension fund, hold none.