Epidemiological research has firmly established the association of social context with late-life health and functioning (Cacioppo et al., 2006
; Cohen, 2004
). Yet this research unambiguously explicates neither the basis for these associations nor how social context relates to the biological and genetic factors known to contribute to later life functioning (Cohen & Janicki-Deverts, 2009
; Hawkley & Cacioppo, 2003
). Several longitudinal twin samples, located in different countries and employing somewhat different measures, have individually accumulated substantial data. Because recruiting and maintaining long-running studies of older adults is made especially difficult by participants' relative frailty, these samples have generally been too small to reliably detect many of the kinds of effects likely to be of interest. Consequently, a collaboration among eight existing longitudinal twin and family studies was initiated, with a central focus on determining how social context is related to physical functioning (health, functional ability), and psychological functioning (well-being, cognition) in mid-life and older ages. Through this, we hope to lay the foundation for future studies of gene-environment interplay in late-life functioning. In the following we describe the rationale and design of the Interplay of Genes and Environment across Multiple Studies (IGEMS) consortium.
With their rich registry systems, the Nordic countries have pioneered many of the major longitudinal twin studies of aging, including the Longitudinal Study of Aging Danish Twins (LSADT) in Denmark (Christensen et al., 1999
) the Swedish Adoption Twin Study of Aging (SATSA; Finkel & Pedersen, 2004
), Origins of Variance in the Old-Old (OCTO-Twin; McClearn et al., 1997
), and Ageing in Women and Men: A Longitudinal Study of Gender Differences in Health Behaviour and Health among Elderly (Gender; Gold et al., 2002
) projects in Sweden. Moreover, investigators in both countries have extended their investigations to younger adult twin samples (e.g., the Middle-Age Danish Twin [MADT] study; Osler et al., 2008
) and Twin and Offspring Study in Sweden (TOSS; Neiderhiser & Lichtenstein, 2008
), to provide foundations for longitudinal investigations of the influence of earlier developmental periods on aging outcomes. While undertaking similar longitudinal twin research in the United States has been more logistically challenging, members of IGEMS have established analogous studies in the United States, most notably the Minnesota Twin Study of Adult Development and Aging (MTSADA; Finkel & McGue, 1993
) and the Vietnam Era Twin Study of Aging (VETSA; Kremen et al., 2006
). Collectively, these studies include over 17,500 participants (including nearly 2,600 monogygotic [MZ] and 4,300 dizygotic [DZ] twin pairs and over 1,700 family members) whose ages ranged from 25 years to over 100 years at their intake assessments. Participants in these studies have undergone multiple longitudinal assessments (median follow-up period = 9.5 years) that include a rich variety of measures of aging-relevant outcomes in three broad domains: physical health and functional ability, psychological well-being (emotional stability/depression), and cognitive health. These studies also include multiple indicators of the social environment, spanning early childhood through late adulthood, and encompass participants with several different environmental contexts, such as differences in healthcare and retirement systems. The combined result is a set of longitudinal studies with assessments of outcomes and social contexts spanning mid- to late-adulthood.
The heuristic model that guides our investigation of gene-environment interplay in late-life functioning is presented in . The model emphasizes the importance of two gene-environment processes. First, we hypothesize that environmental exposures and social contexts do not occur at random, but rather reflect an individual's genetically influenced behavior and choices (i.e., gene-environment correlation). We know, for example, that there are heritable influences on social factors including social support (Bergeman et al., 2001
), social engagement (McGue & Christensen, 2007
), and social isolation (Boomsma et al., 2007
). We further hypothesize that gene-environment correlations are a major contributor to late-life phenotypic stability. For example, based on Schmalhausen (1946)
, we hypothesize that a shift from being socially engaged and physically active in middle age to being socially isolated in late-life will be accompanied by marked discontinuities in behavioral stability. Data in the IGEMS consortium allow us to chart the contributions of changes in social context to behavioral discontinuities; the twin-study design allows us to further explore the nature of gene-environment interplay in these transitions.
(Colour online) Heuristic model forming basis of investigation of gene-environment interplay in late-life functioning.
Second, we hypothesize that experiential factors can either diminish or amplify the influences of genetic effects on late-life outcomes (genotype-environment interaction, G × E). Two of Shanahan and Hofer's (2005
) four alternative models of G × E are directly relevant. Evolutionary models (Schmalhausen, 1946
) predict that the G × E form termed Contextual Triggering
by Shanahan and Hofer, but recognized more broadly as the diathesis-stress model, will be more pronounced in late life. Specifically, during most of life, the typical human lives in an environment that is low in stress relative to the environments in which their species evolved (Owens, 2002
). In old age, biological, psychological, and social stresses are considered to be higher due to more negative change or declines. Thus, at times of relatively increased stress—such as old age—an organism's response to an unfavorable environment will depend on a unique combination of genes, so that genetic effects on physical manifestations of health will be maximized (Schmalhausen, 1946
). This pattern of findings is supported by earlier work by IGEMS investigators, showing that the heritability of physical health in mid-life is maximized when current income (Johnson & Krueger, 2005a
) and perceived life control (Johnson & Krueger, 2005b
) are low.
Alternatively, environmental factors can suppress genetic effects through a form of G × E Shanahan and Hofer call Social Context as Social Control. We predict that the Social Context as Social Control form of G × E is particularly relevant to extreme forms of physical disability or disease, which diminish the impact of genetic factors by reducing the opportunity for self-selection of activities that maintain stable functioning.