"What Do The Best-Performing Stocks in 2018 Have In Common?"

A twofer, first up the headline story.
From Ivanhoff Capital, March 13:

20 stocks have more than doubled year-to-date.

Many are biotech, but this is a cyclical, not a structural reason. A structural reason is one that persist; one that shows over and over again. 

All of them had a market cap of under $1 Billion on January 1st. 

All of them have a float of under 100 million shares. Most have a float of under 50 million shares...MORE

And pulling back for a longer term view, again Ivanhoff Capital, January 24:

The Three Best Performing Stocks for the Past 15 Years Will Surprise You

Netflix recently hit new all-time highs and it ended up on the first page of many newspapers. 10,000 invested in Netflix’s IPO in 2002 is worth about $2.3 Million today. This amounts to about a 40% average annual appreciation.

As impressive as NFLX’s return is, it is not even the best-performing stock for the past fifteen years. Here are the top three. They are all consumer stocks – a Chinese video game maker, a U.S. energy drinks producer, and a U.S. video content creator and distributor.

NTES +11,706%
MNST +77,230%
NFLX +23,467%

...MUCH MORE