Educability-and-Group-Differences-1973-by-Arthur-Robert-Jensen

Intelligence and Educability 87
used as the base from which gains were computed.) This finding
is consistent with the simplex model. Very little of the gain in a
year’s interval becomes consolidated as status. If it did, we should
expect a much higher correlation between independent measures
of status and of gain. Moreover, if a large random element did not
enter into the short-term gains we should expect consistent
individual differences in gains from one interval to the next and
consequently substantial correlations between gains from one
interval to the next. But this in fact is not the case. Thorndike
gives the average correlation between two independent gain scores
on intelligence tests for different intervals:
1-year interval = 0-101
2-year interval = 0-240
3-year interval = 0-266
4-year interval = 0-188
5-year interval = 0-265
The longer the interval, of course, the larger is the proportion of
the gain that has been consolidated and therefore the larger the
correlations between gains over longer intervals. The same effect
is reflected in the average correlations of initial status with gain
based on experimentally independent tests:
1-year interval = 0-045
2-year interval = 0*006
3-year interval = 0-031
4-year interval = 0-139
5-year interval = 0-329
These actual correlations are even smaller and somewhat less
regular than would be predicted from the simplex model, probably
because of measurement error, slightly changing factorial composition
of the tests at different levels of difficulty (and thus at different
ages), and unequal units of measurement over the full range of
scores.
Another fact to be considered in this model is the heritability of
the trait under consideration. This is quite high for intelligence
and vocabulary, but lower for scholastic achievement, particularly
in the elementary grades and for subjects such as spelling and
mechanical arithmetic. Of all the growth characters on which
there are good data, the highest heritability is for height. What