Too Big to Co-Move
The ten largest firms rose from 24% to 41% of S&P 500 market capitalization between 2018 and 2025, and by August 2026 connectedness among the eleven sectors had reached its lowest level in the sample. Does concentration explain the disconnection?
Concentrated sectors co-move less with the rest of the market: a one-point rise in top-three share lowers monthly R² by 0.75 points (permutation p = 0.04), though concentration does not explain why markets disconnected.




