 15. Personality Development in Adulthood and Later Life  347
ferences in intraindividual change at its cen -
ter. Darwin, in evolutionary theory, placed 
individual variation in physical traits at the 
center of natural selection. This is not to say 
that either theory does not acknowledge the 
idea of a universal general design. Both do. 
As noted by McAdams (2015), all humans 
share a general physical form, but there are 
infinite variants on that form. Personality 
science, and the science of personality de -
velopment, is uniquely able to acknowledge 
a general principle (e.g., social investment), 
along with a parallel acknowledgment of 
the individual differences that surround that 
general pattern. Not everyone will adhere to 
a given theoretical concept, or adhere to it in 
the same manner. This brings us full circle, 
back to Kluckhohn et al. (1953), and their 
dictum that in some ways, each one of us 
is like no others. In the realm of adult per -
sonality development, one major way this is 
manifested is in the unique trajectory each 
person possesses, irrespective of the shape of 
the population metatrajectory. A theory can 
generalize across people only so far. Once it 
reaches its boundary, individual differences 
remain. This notion forms the core of per -
sonality science and of the study of adult 
personality development.
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