social factors to health outcomes. The model complements and extends previous biopsychosocial models, positing a common element among them: an imbalance between the branches of the autonomic nervous system (ANS), the parasympathetic and sympathetic, that favors the latter. The status of the ANS at any time is a function of the inhibitory influences of the parasympathetic system and the excitatory effects of the sympathetic system. The ANS is important since it controls cardiovascular, neuroendocrine, and immune system functions; it also interacts with key brain regions related to psychological and emotional processes. Thayer and Friedman (2004) hypothesize that the lessening of parasympathetic influence in the ANS (as indexed by heart rate variability), especially under psychosocial challenges such as stress, worry, perseverative thinking, depression, anxiety, racism, and low status, may be a key biological pathway that underlies racial and ethnic differences in health in late life.

Although this model has not been evaluated with respect to health differences, there is evidence that lowered heart rate variability and a higher resting heart rate (as an index of decreased parasympathetic tone) are related to increased all-cause mortality (Thayer and Friedman, 2004).

These biobehavioral models of disease should not be viewed as competing conceptualizations, since there is considerable overlap among them. Cardiovascular reactivity, allostatic load, immune system changes, metabolic syndrome, and heart rate variability are strongly correlated. At this time, it is not clear which of these, or any other biobehavioral model, is most predictive of illness and death. Nevertheless, these models are likely to be related to many of the proposed determinants of health differences, such as stress, social support, racism, health risk behaviors, and socioeconomic status, and may therefore indicate mechanisms through which such risk factors translate into disease susceptibility and health differences.

NEEDED RESEARCH

Much basic research is still needed on biopsychosocial mechanisms, and further work is needed to elucidate the links, if any, with race and ethnicity.

Research Need 13: Clarify how biopsychosocial factors affect health outcomes over time in racial and ethnic groups of middle-aged and older adults.

It has been hypothesized that psychosocial effects on biopsychosocial outcomes, such as cardiovascular reactivity, allostatic load, psychoneuroimmunology, metabolic syndrome, and neurovisceral integration are predictors of morbidity and mortality. It remains an empirical question whether



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