As investors turn to index-trackers in ever-greater numbers, funds offering targeted bets on industries such as robotics have proved a lucrative lifeline for active stock-pickers. But their performance has mostly failed to impress, according to a new report.

Assets invested in so-called thematic funds, which aim to pick the stocks best set to profit from trends such as technological development, rising longevity or water scarcity, have grown quickly — from $75bn at the end of 2016 to $195bn by the end of last year, according to the report from Morningstar, the fund analytics firm.