LIGO/Virgo have reported six binary black hole ( BH-BH ) mergers .
The effective spins of all of them are clustered around \chi _ { eff } \sim 0 .
However , the effective spin of one of them , GW170104 , has an 82 \% probability of being negative , which would indicate a significant spin-orbit misalignment and seemingly supporting dynamical formation over isolated binary evolution .
We show ( i ) as a proof-of-principle case , that GW170104 could have been formed through isolated binary evolution , and ( ii ) that the LIGO/Virgo measurements inform about , thus far unconstrained , angular momentum transport in massive stars .
Massive stars can have inner transport of angular momentum and their strong winds may carry away substantial angular momentum , affecting the natal spin of the BHs created at the end of their lives .
We present a physically motivated model for BH natal spins based on moderately efficient angular momentum transport .
With this model we can explain the low effective spin of GW170104 in the context of isolated binary evolution .
However , our predicted effective spin distribution of BH-BH mergers , showing a large fraction of high effective spins , is not consistent with LIGO/Virgo observations .
If the effective spins of BHs observed by LIGO/Virgo continue to be small , it will indicate that angular momentum transport in massive stars is more efficient than adopted in our model and that BHs found in BH-BH mergers are born with low spins ( a _ { spin } \sim 0 ) .