  We study the one-loop radiative corrections for massless fermions in de
Sitter space induced by a Yukawa coupling to a light, nearly minimally coupled
scalar field. We show that the fermions acquire a mass. Next we construct the
corresponding (nonlocal) effective fermionic action, which -- in contrast to
the case of a massive Dirac fermion -- preserves chirality. Nevertheless, the
resulting fermion dynamics is precisely that of a Dirac fermion with a mass
proportional to the expansion rate. Our finding supports the view that an
observer or a test particle responds to a scalar field in inflation by shifting
its energy rather than seeing a thermal bath.
