This is a good paper explaining the theory - HFT concentration is high & increasing as barriers to entry become insurmountable for new HFTs: https://poseidon01.ssrn.com/delivery.php?ID=623110115067019001070028025111115092051040069008061028095107010090009064077122029071123059021013106005121065002103064096085029105043041005072083111005120013028117096056082040091094089070092102003122097082010028064104003075031025018119084004021113001087&EXT=pdf&INDEX=TRUE …pic.twitter.com/0VsHKbHB95