  This article brings attention to some historical developments that gave rise
to the Bayes factor for testing a point null hypothesis against a composite
alternative. In line with current thinking, we find that the conceptual
innovation - to assign prior mass to a general law - is due to a series of
three articles by Dorothy Wrinch and Sir Harold Jeffreys (1919, 1921, 1923).
However, our historical investigation also suggests that in 1932 J. B. S.
Haldane made an important contribution to the development of the Bayes factor
by proposing the use of a mixture prior comprising a point mass and a
continuous probability density. Jeffreys was aware of Haldane's work and it may
have inspired him to pursue a more concrete statistical implementation for his
conceptual ideas. It thus appears that Haldane may have played a much bigger
role in the statistical development of the Bayes factor than has hitherto been
assumed.
