Because the law for the most part consists of a system of rules for th
e control of human social behaviour, the law may be taken as a branch
of applied psychology. However, one may well ask how good lawyers are
as applied psychologists. As a partial answer to this question two wel
l-known issues in criminal law are explored; the deterrent effect of p
unishment and the viability of the presumption of innocence. It is arg
ued that lawyers, although they tend to speak in confusing ways about
the deterrent effect of punishment, in practice show themselves to be
pretty good applied psychologists. This is different with respect to t
he viability of the presumption of innocence, as they do not appear to
realize that the ways in which criminal trials are commonly conducted
cannot but violate this sacred principle of due process.