4 and estrogen have even been found in mitochondria of diﬀerent cell types [90, 91]. Acutely, these “stress” hormones increase mitochondrial biogenesis and function [87]. How- ever, chronic exposure to elevated levels of these hormones, which can be induced by psychosocial stressors (e.g., social isolation, depression, and violent or abusive social environ- ment) [93], can lead to reductions in mitochondrial mass (see Figure 1) and concomitant increases in mitochondria- derived ROS production [87, 92]. These mitochondrial outputs synergistically damage cellular components and con- tribute to cellular senescence when chronically produced at high levels. Beyond the direct eﬀects that psychosocial stresses exert on mitochondrial function, psychosocial factors can also inﬂuence individual’s lifestyles, such as levels of physical activity and inactivity (i.e., sedentariness) [93]. For example, negative perceptions of one’s body image and negative inﬂuence from family and friends are associated with lower levels of physical activity [95–97]. Similarly, people who are depressed or suﬀer from mental illnesses also tend to be more physically inactive [98, 99]. Physical inactivity can in turn undermine physical and mental health [100], predis- posing inactive young individuals to depression later in life [101]. Physical activity has positive eﬀects on mitochondrial function and counteracts inﬂammatory processes and age- related chronic diseases [55]. It can even buﬀer against the negative eﬀects of chronic stress on telomere shortening [102]. In fact, the beneﬁcial eﬀects of physical activity and exercise on the hormonal system (e.g., increases in inter- leukin 6, growth hormone, brain-derived growth factor— BDNF) [103, 104], on psychological and cognitive aspects (e.g., decrease in stress levels and reactivity to stress, depres- sion, improved well-being) [105], as well as on metabolic regulation (e.g., increased mitochondrial mass and improved insulin sensitivity) [65, 106], suggest that exercise and physical activity exert multisystemic protective eﬀects which can prevent the deleterious consequences of chronic stress [107]. Indeed, improving physical ﬁtness has been shown to decrease hormonal, physiological and psychological markers of chronic stress [72]. Psychosocial factors and physical activity can therefore interact to inﬂuence mitochondrial function and modulate the impact of chronic stress on the body. Because mitochondria inﬂuence cellular aging and are responsive to stress hormone levels, they are especially well equipped to act as key integrators that synergistically inﬂuence biological and psychosocial factors (Figure 1). As described above, work in the psychosocial sciences has unravelled important links between how individuals feel, their social contexts, and the eﬀects of these factors on mortality and longevity. However, how these factors inﬂuence and interact with biological factors remains to be explored in more depth. The ﬁndings described above and many others bring new evidence coaxing researchers to focus on the interactions of biological and psychoso- cial forces that inﬂuence the aging process [70]. Conclu- sions derived from research not adopting an integrative approach risk being uni-dimensional and thus diﬃcult to Journal of Aging Research apply towards diﬀerent real-life contexts, where individuals age under the collective inﬂuence of factors of diﬀerent nature. 5. Interdisciplinarity: A Necessary Framework for Aging Research? Interdisciplinarity and even transdisciplinarity [108] have emerged as key necessities in the ﬁeld of aging and others [108–110]. Both the National Institutes of Health (NIH) in the USA and the Canadian Institutes of Health Research (CIHR) funding agencies have opened institutes on/of aging promoting broad mandates that necessarily reach across tra- ditional disciplinary boundaries. Although some problems are best addressed with the approach of a single discipline, other issues require the integration of several disciplines to fully comprehend the complexity of the processes at play [111]. This is particularly true for aging [109]. Likewise, the discipline of developmental psychopathology, driven by the impetus to grasp and impact the complexities of mental health at diﬀerent stages of development, has evolved to become a deeply “interdisciplinary ﬁeld that seeks to elucidate the interplay among the biological, psychological, and social-contextual aspects of normal and abnormal development across the life course” [112, p.16]. The writings of Cicchetti and Toth [112] are particularly enlightening regarding the conceptual, theoretical, and practical direc- tions to take to achieve such a degree of integration within a discipline. They particularly stress the importance of cross- disciplinary dialogue [112], along with the need for an emphasis on the process of development naturally resulting from the interdependence over time of multiple biological and psychosocial factors [113]. In this spirit, the aging process is inﬂuenced by several interactive forces inherent to the individual (e.g., genetic endowment, physical activity, diet, lifestyle, and personality) and forces inherent to the psychosocial environment (e.g., sociocultural context, family, and life stress) that continu- ously and progressively interact over long periods. Aging must therefore be approached from an ontogenic perspec- tive not solely focused on end outcomes (i.e., mortality, comparison of aged and young individuals) or on the molecular factors predicting these outcomes. Instead, aging research would beneﬁt from being guided by a perspective focused on the changes and interactions among biological and psychosocial processes, which take place across stages of human development throughout the lifespan. Table 1 lists ﬁve conceptual propositions elaborated by Ryﬀ and Singer [70] to advance hypothesis-testing research focused on healthy aging as an interdisciplinary process. In a similar way that mental health and disease emerge from cumulative life experiences in infancy, childhood, and adulthood [112– 114], aging must also be a deeply experience-dependent process where the biology inﬂuences how a person responds to their experiences, but where the biology is also shaped by those experiences.