This speculation could have significant knock-on effects. "These are all what I would describe as typical signs of excess and definitely things that make us take a cautious view on the overall market," Russell said. "What's difficult to really put across is the fact that this excess is pretty much contained into large, but specific, pockets of the market." Though the overall index could fall, it doesn't necessarily mean everything will go down in value, Russell said. Russell is still investing in traditional businesses, such as retail, energy and financial companies, while avoiding momentum stocks, the FAANGs, as well as any business where investors are being asked to pay high multiples for future profits.