In a severe bear market, fear spreads like wildfire. The latest worry is that plummeting stocks might disappear off the screens completely. One requirement for membership of the S&P 500 index is a market capitalisation of more than $4bn. At Friday’s opening prices, the index would have had to be renamed the S&P 256. But investors need not panic. Unlike indices that reshuffle stocks automatically, the S&P 500 is maintained by a committee. Index membership is based on a number of criteria, some, or indeed all, of which can be overlooked by the committee, which strives to minimise movements in and out of the index. Turnover last year, for example, was just 5 per cent. The minimum market capitalisation rule, for example, can be adjusted to reflect market conditions. That is just as well: the smallest S&P stock is worth just $150m.