One of the biggest talking points for investors in the past year or two has been the increasingly concentrated nature of the US sharemarket. The top ten stocks represent a whopping 38 per cent of the index today, compared with 20-30 per cent historically. A few mega-cap stocks now have a huge influence over where the world’s biggest market (and therefore most of our retirement savings) is headed and unsurprisingly, this has made a lot of investors nervous.