samples and different measures, conducted at different times. The hope
is that all these influences will balance out, leaving a good estimate ofthe true value. But there is another, arguably better, way to resolve thequestion of when personality stabilizes: Compare retest correlations foryounger and older samples measured at the same times and with thesame instrument. We have done that with the GZTS (Costa et al., 1980)and with the NEO-PI (Costa & McCrae, 1988b), and we have consis -
tently found that men and women in their 30s and 40s show stability
coefficients as high as men and women in their 50s and 60s. Table 13 il -
lustrates this fact: The median stability coefficients for the younger
groups are .74 for both men and women; for the old group, they are .75for men and .69 for women. Already at age 30, men and women appearto show extremely high consistency over time in their personality traits.
***
The idea that personality should be so deeply and permanently in-
grained was revolutionary in a scientific climate in which change,growth, and development were the watchwords. Not surprisingly , there-fore, a number of psychologists challenged the basic findings when theywere first published.
When unanticipated results are found, there is a strong tendency to
dismiss them. This reluctance to accept an unforeseen result is not re-ally a result of closed-mindedness, nor is it unscientific. On the con-trary , the essence of science is to look critically at observations and con -
clusions. Every scientist knows that there is more than one explanation
for any phenomenon and is rightfully wary of accepting the first ac -
count that offers itself. True, the high correlations between personality
scores at two different times are just what one would expect if personal -
ity is actually highly stable. But there are a number of other ways in
which the same correlations could be observed that are entirely consis -
tent with major change in personality . Until these rival hypotheses can
be ruled out, it is premature to accept the conclusion of stability .The Course of Personality Development 115