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IT IS now ten years since the three authors agreed to
pool their intel-
lectual resources in an attack upon the much-debated
problem, using twins as the most favorable weapons for
such an at-
tack. It was thought that the three of us together a
psychologist, a
statistician, and a specialist in the biology of twins
might be able to go
more deeply into this problem than could one person
with one particular
type of training.
Originally the plan was to compare, in as many ways as
we could, a group
of identical twins reared together with an equivalent
group of fraternal
twins reared together. Subsequently, when we began to
secure for study a
number of pairs of identical twins reared apart, it
seemed advisable to
postpone an attempt to arrive at final conclusions
until the number of
twins reared apart became sufficiently large to have
statistical value.
These cases came in slowly, and it was only a little
over a year ago that we
decided that the time had come for publishing a general
report of the whole
In this study there has been continuous collaboration,
each of us con-
tributing his own type of specialized knowledge to each
part of the study,
in addition to participating in making the general plan
of the investigation.
Dr. Freeman has had charge of all the psychological
tests; Dr. Holzinger
has been responsible for the statistical aspects of the
work, including
tabulation and analysis of data, and the derivation of
the formula for the
estimate of the relative effect of nature and nurture,
and has also par-
ticipated in the collection of data on twins reared
together; Dr. Newman