Momentum stock downturn last week

Jim Cramer made the point that momentum stocks are a sector by themselves irrespective of what conventional industry they are in under SP500 / NASDAQ. When they go down, they go down together. You can have a part of your portfolio in them, but not too much. They were hit pretty hard on days like Thursday or Friday last week. 

Sure, your portfolio sinks, but these days, it would mess with your confidence and temperament. He worries that many investors will withdraw completely from investing as a result. And that's bad for the retail investor revolution that we are in.