years. Analyses showed that there were significant declines in N, E, and
O, but they were tiny: about one-tenth of a standard deviation. At thatrate, over the 40 years from 30 to 70, N, E, and O would decline lessthan half a standard deviation—an amount in line with the very smallcross-sectional declines seen in Figure 2. Longitudinal declines in Nand E were also reported by Loehlin and Martin (2001) in a largeAustralian sample using a different measure. With large samples, it ispossible to see longitudinal changes that mirror the cross-sectional agedifferences.
The NHANES sample did not include measures of A and C, but
later cross-sectional studies (Costa & McCrae, 1992a) suggested thatthese two increased with age. Certainly it is the case that college stu -
dents are less conscientious and agreeable than adults, and so far devel -
opmental changes in later adulthood have echoed the more substantial
changes in early adulthood. But the Costa, Herbst, et al. (2000) studydid not support this hypothesis. Agreeableness did not change at all,and Conscientiousness actually showed a very small decline. As weshow in a later chapter, there is an entirely different line of evidence thatsuggests that A and C really do increase across the adult years. It is fairto say that the developmental course of A and C after age 30 is currentlya mystery—except that whatever changes occur are probably small andsubtle.
Ravenna Helson has devoted her career to long-term longitudinal
studies. Most of her samples have been relatively small, but by now shehas followed some of them for as long as 40 years. In a series of analysesof CPI scales over that time interval she demonstrated significantchanges consistent with decreases in N and E and increases in C(Helson & Kwan, 2000). These longitudinal changes averaged aboutone-third standard deviation. At least with respect to N, E, and O, thereis now fairly consistent evidence of decline during adulthood.
How small a change is one-third standard deviation? One way to
think of it is by a comparison with height and weight. The standard de -
viation of height among adults is about 3½ inches, and one-third of that
is a little more than 1 inch. So if height declined at the same rate asNeuroticism, a six-foot-two 30-year-old could expect to be six-foot-oneat age 70. That is a perceptible decline but hardly a dramatic one: Six-foot-one is still tall in an absolute sense. The comparison with weight iseven more revealing, because most of us are acquainted with changes inweight. The standard deviation of weight among adults is about 35Growth or Decline in Personality 79