Primordial neutrinos decoupled in the early universe in helicity eigenstates. As I will discuss, two effects -- dependent on neutrinos having a non-zero mass -- can modify their helicities as they propagate through the cosmos. First, finite mass neutrinos have a magnetic moment and thus their spins, but not their momenta, precess in cosmic and galactic magnetic fields. The second is the propagation of neutrinos past cosmic matter density fluctuations, which bend their momenta, and bend their spins to a lesser extent. Both effects turn a fraction of left-handed neutrinos into right- handed neutrinos, and right-handed antineutrinos into left-handed. If neutrino magnetic moments approach even a fraction of that suggested by the XENON1T experiment as a possible explanation of their excess of low energy electron events -- a value well beyond the moment predicted by
the standard model -- helicities of relic Dirac (but not Majorana) neutrinos could be considerably randomized. I finally will discuss the implications of neutrino helicity rotation, as well as their Dirac vs. Majorana nature, on their detection rates via the Inverse Tritium Beta Decay reaction. This work is summarized in two recent papers, G. Baym and J. C. Peng, Phys. Rev. Letters 126, 191803 (2021) [arXiv:2012.12421v3 [hep-ph]]; and G. Baym and J. C. Peng, Phys. Rev. D 103, 123019 (2021) [arXiv:2103:11209[hep-ph]].