 37. Personality and Health 777
experimenting with drugs during this criti -
cal period (Chambers, Taylor, & Potenza, 
2003). The concept of critical periods has 
evolved from being defined strictly by age to 
looser phases that occur in a maturational 
sequence (Michel & Tyler, 2005).
Accumulation  Models
Accumulation models emphasize that the ef -
fects of risk exposure build progressively over 
the life course. Insults (e.g., the pollutants in 
a low-  income neighborhood) build and inter -
act with other risks, gradually increasing the 
threat to health. A pioneering elaboration on 
this idea was Caspi, Elder, and Bem’s (1987) 
cumulative continuity  model, in which child -
hood personality has the effect of channeling 
the individual into a particular set of envi -
ronments, which results in an accumulation 
of consequences. Relatedly, in interactional 
continuity,  personality traits influence an 
individual’s interaction style, which in turn 
evokes a style of responding from others that 
may serve to perpetuate that style, even if it 
is ultimately maladaptive.
A type of accumulation model applied 
to stress is the allostasis model (McEwen, 
1998). Allostasis  is the ability to achieve sta -
bility through change. Throughout ongoing 
challenges, the body uses the autonomic ner -
vous system, the HPA axis, and the cardio -
vascular, metabolic, and immune systems to 
respond to threats. Importantly, the “stress 
response” is a component of resilience and 
not necessarily negative (Karatsoreos & 
McEwen, 2013). We cannot live without 
it, and cortisol levels and immune markers 
should  be high at certain times. It is repeated 
and relentless stress and coping that is ul -
timately damaging to the integrity of these 
bodily systems.
Critical periods and accumulation need 
not be independent. Risk effects may both 
accumulate over the life course and be great -
er during critical periods. Combining these 
epidemiological models with the traditional 
models of personality and health (described 
earlier in this chapter) provides a deeper 
analysis of mechanisms. It encourages us to 
consider how the intervening mechanisms of 
health behaviors, or physiological processes 
or situation selection, have a cumulative in -
fluence or are disproportionately influential 
during particular stages.Lifespan  Research
A striking example of the richness of a life -
span perspective involves the influence of 
gender on health. One of the most robust 
findings in epidemiology is that women out -
live men. All sorts of distinct explanations 
for this difference have been proposed and 
investigated, but none has been definitively 
confirmed, suggesting the complex interac -
tive effects of biology, personality, and so -
cial relations. In addition to physiological, 
anatomical, and maturational differences 
relevant to personality and health, there 
are many constantly changing sociocultural 
differences in the lives of men and women. 
Gender is a central variable in any lifespan 
approach to personality and health, and 
where feasible, gender differences should al -
ways be examined.
Individuals cannot be randomly assigned 
to a particular personality, gender, culture, 
or psychosocial environment; that is, per -
sonality and health research must, by neces -
sity, use methods common in epidemiology 
and developmental psychology, with long-
term prospective research designs being 
the most informative. Pieces of the puzzle 
sometimes can be subjected to experimen -
tal analysis, such as in randomized trials of 
behavior modification, or of stress-  relevant 
pharmaceuticals, or studies involving labo -
ratory animals. However, such experiments 
rarely capture the full range of interacting 
influences on subsequent health. So it is im -
portant to remain aware of the temptation 
to overinfer causality. For example, religious 
people and married people are generally 
healthier, but this does not imply that we 
should urge religious observance or mar -
riage to improve health, indiscriminately at 
the level of the whole population.
Origins in  Infancy and  Childhood
The brain and the rest of the nervous and 
neuroendocrine systems are at the center 
of the relations between personality and 
health, and their development sets the stage. 
The most rapid development begins a few 
weeks after conception and continues at a 
significant pace until about age 5, at which 
point a recognizable personality and self- 
identity has formed. By age 5, most children 
have a gender identity, and personality and 