  We show that Landau theory for the isotropic, nematic, smectic A, and smectic
C phases generically, but not ubiquitously, implies de Vries behavior. I.e., a
continuous AC transition can occur with little layer contraction; the
birefringence decreases as temperature T is lowered above this transition, and
increases again below the transition. This de Vries behavior occurs in models
with unusually small orientational order, and is preceded by a first order I
&#8722; A transition. A first order AC transition with elements of de Vries
behavior can also occur. These results correspond well with experimental work
to date.
