  Born's original 1933 theory of nonlinear electrodynamics (in contrast to the
later Born-Infeld theory) is acausal for strong fields. We explore the issue of
strong-field causality violation in families of theories containing Born and/or
Born-Infeld, and many variants that have been previously proposed in contexts
that include cosmology and black hole physics. Many of these variants are
acausal and hence unphysical. A notable exception is the modified Born-Infeld
theory with ModMax as its conformal weak-field limit.
