784 vII. APPLICATIONS  AND  IMPLICATIONS
vin et al., 2011; Deary, Whiteman, Starr, 
Whalley, & Fox, 2004; Gottfredson & 
Deary, 2004; Hart et al., 2003; Pavlik et 
al., 2003). Although not usually conceived 
to be an element of personality, intelligence 
is certainly a key individual difference with 
long-term stability, and with influences on 
many life key outcomes. Some of the same 
factors that produce an association between 
conscientiousness and health also lead to an 
association between intelligence and health. 
Lower childhood IQ is a risk factor for 
health-  damaging behavior in adolescence 
and adulthood. Cognitive impairment pre -
dicts and is predicted by psychophysiologi -
cal disruptions such as hypertension, obe -
sity, diabetes, brain integrity, and incipient 
cardiovascular disease. Lower intelligence is 
correlated with and predicts less educational 
achievement, lower SES, less adherence to 
medical treatment, and less stable social re -
lationships. On the other hand, the Terman 
cohort indicated that a large percentage of 
highly intelligent individuals succumb to un -
healthy behaviors and diseases in patterns 
that are not substantially different from 
those of many persons of average intelli -
gence, indicating the importance of person -
ality traits and other factors in addition to 
intelligence for health outcomes (Friedman 
& Markey, 2003). Thus, it remains to be ex -
amined just how personality and intelligence 
interact in the prediction of health outcomes 
at various stages of the life course (Iveson & 
Deary, 2019).
Conclusions and  Future  Directions
Because understanding the likelihood of dis -
ease for the individual is often as important 
as knowing the general causes of disease, 
personality psychology is a necessary com -
plement to biology in understanding and 
promoting health and longevity. Modern 
conceptions of personality are addressing 
many of the limitations of static conceptions 
of health, although these new developments 
are underappreciated in medical research 
and treatment. Personality concepts and 
theories are comfortable with interacting 
biological, psychological, and social con -
tributions, as well as changes as a function 
of maturation, environmental press, situ -ation selection, and cultural influence. For 
example, whereas personality psychologists 
are comfortable with describing a worry -
ing, impulsive, working-  class boy with bio -
logically based hyperreactivity, who grows 
up exposed to a high-  carbohydrate diet re -
inforced by certain social gatherings that 
are part of his cultural environment, and 
who sets himself, during adolescence, on a 
social–  behavioral pathway toward diabetes, 
a physician might struggle to identify which 
“medical condition” condition to “treat.” 
Thus, although there is no pill to treat this 
progression (and no pill to change personal -
ity), there are many positive steps that can 
be taken to head off serious chronic illness.
To illustrate the lifespan approach, we 
have examined the relations between per -
sonality and health in terms of three do -
mains: (1) effortful control and conscien -
tiousness, (2) negative emotionality, and 
(3) positive emotionality and optimism. All 
three domains are proving fruitful in the 
search for links between personality and 
health, but these do not necessarily repre -
sent an optimal categorization. Although 
health–  behavior models tend to be invoked 
as mechanisms to account for the effects 
of the conscientiousness, and stress-  related 
psychoneuroimmunological models histori -
cally tend to be used to explain the effects of 
emotionality, more complete models will in -
tegrate the various interacting forces, while 
considering changes across time (Friedman, 
Kern, Hampson, & Duckworth, 2014).
To test lifespan models, we need to em -
ploy statistical techniques that can optimal -
ly analyze trajectories such as latent-  growth 
models and latent-  transition analysis. This 
methodology requires multiple, reliable 
assessments over the longitudinal period 
under study. Relatedly, survival analysis is 
standard in epidemiology but only slowly 
is making its way into personality psychol -
ogy. These methods allow for personality 
change, as well as changes in health status. 
As noted, it also should prove valuable to 
identify life stages—  the critical periods— 
when both personality and health are more 
likely to change with regard to particular 
habits, behaviors, social ties, reactions, and 
health status.
Because the relations are dynamic and 
complex, there may always be limits on the 