Turnover on corporate boards remains low as directors stay in their seats for more than a decade on average, according to a new study from a business think tank.

About half of the companies in the S&P 500 and Russell 3000 indexes disclosed no changes in their board makeup in 2018, a similar turnover rate to 2016, the study released April 24 by the Conference Board found. Boards that changed their membership typically turned over just one seat.

Many public...