The S&P 500 has been making headlines this summer, posting its longest ever bull market despite growing volatility elsewhere in the world. Indeed, it remains on course to beat other developed-market equity indices by more than 5% for the second successive quarter. However, it is US small caps that have attracted our attention. In this In Depth, we look at the strong recent performance of US small-cap equity indices, and why we think this trend might be facing some headwinds