Reserves

Initially developed in neuroscience, the reserves concept was use to study the level of protection against cognitive damage (see “cognitive reserve”, e.g., Stern, 2009). In its extended understanding proposed by Cullati, Widmer and Kliegel (2018), the reserves concept can refer to all areas of life and must allow to better grasp vulnerability processes across the life course. In this context, reserves are defined as « the means needed not for immediate use but rather accumulated in a sufficient manner. Such means help overcome shocks and adversity and delay or modify the processes of decline in well-being, health, wealth and social life during aging. » (Cullati et al., 2018, p. 551). In other words, reserves refer to the various means available to individuals to face distributive events or transitions during life course. With respect to « resources », the notion of reserves highlights the time-oriented dimension of the means available to individuals to avoid or deal with vulnerability by stressing their dynamics of constitution through accumulation, their processes of activation across life course, but also their risk of depletion when facing a critical life event, and possible need for reconstitution after depletion. The thresholds under which reserves cannot be transform into effective resources to face vulnerability are another issue raised by the reserves concept that is important in understanding vulnerability dynamics.

Constitution : passive and active
The constitution of reserves may refer to passive or active process (Cullati et al., 2018). Passive process of reserves constitution refers initially, regarding its origin in neurosciences, to innate (i.e. genetically determined) and very early acquired (typically through family environment or with school education) inter-individual differences that impact trajectories of cognitive functioning. In a more social sciences perspective, passive process refers also to social inequalities or the impact of social background on further life course conditions. Conversely, Cullati and colleagues argue also for an active model of constitution, where initial reserves can be changed across individual trajectories. For instance, reserves for cognition may be enhance through stimulating activities (e.g. Ihle et al., 2018). The development of social ties (e.g. friendship, colleagues, partner) is another example of the opportunities to developpe important reserves across life-course. In the end, taking into account these two principles of reserves constitution – passive and active - seems to be crucial to understand vulnerability and reserves dynamics and to examine cumulative processes of (dis)advantages and trajectories of resilience.

Activation, depletion and reconstitution
While supporting the “active” perspective on reserves, various principles stress the time-dynamic dimension of this concept. The first is the plasticity principle that refers to the “use it or lose it” notion or the fact that activities contribute to the maintenance of the ability to function (Cullati et al., 2018, p. 553). In other word, this implies that reserves need to be activated through life course to be effective when needed; for instance, stimulating activities help preserve cognitive reserve or regular exchange contribute to maintain relational reserves. Another step of reserves dynamic refers to their depletion when facing a critical life event. Indeed, reserves can decrease after their use or become inadequate over life course exigences, involving the need for individual to reconstitute their reserves.
These various principles can vary in intensity among reserves types: for instance, friendship ties have been showed as asking more individual investment in their maintenance (notably through the reciprocity principle)in contrast with family ties whose functioning would be more guided by solidarity norms and involves more automaticity (Allan, 2008). Beyond, historical changes have also an impact of the importance of individual investment in reserves dynamics. Indeed, in western societies after WWII, the standardized life course and its institutions (education, family, retirement), include a relative stable and coherent model of reserves that secure individual life course. With changes toward more destanstandardized society, constitution and activation of reserves, e.g. education or family ties, focus more on individual agency – in other word are less linear, secured and automatic. The more frequent and numerous life transitions and disruptive events that tend to happen during life course increase the use of reserves but also the risk of their depletion and the need of their reconstitution (Baeriswyl et al., submitted).

Threshold
Resources must be accumulated in a sufficient manner to constitute effective reserves. Under a certain threshold, reserves do not allow individual to face adverse life event or life transition and to continue functioning according to societal standards (Cullati et al., 2018). Understanding reserves threshold is a crucial issue for social and health policy in order to establish appropriate measures for compensating the lack of individual reserves.

Various types of reserves
The composition of reserves includes various resources areas : education, social and family relationships, psychological and mental health, income and wealth. Of course, reserves issues vary among its various domain (e.g. expected outcomes depending on life areas or inequalities and individual differences in constitution or activation). Beyond, the various types of reserves and their outcomes interact across life course in complex causality chain – where resources are both outcomes and means of reserves accumulation - that contribute thus to explain individual vulnerability trajectories (Cullati et al., 2018; Spini et al., 2017).

References
Allan, G. (2008). Flexibility, friendship, and family. Personal Relationships, 15(1), 1–16. https://doi.org/10.1111/j.1475-6811.2007.00181.x
Baeriswyl, M., Widmer, E. D., & Oris, M. (submitted). A reserves perspective on education over recent historical time in Switzerland.
Cullati, S., Kliegel, M., & Widmer, E. D. (2018). Development of reserves over the life course and onset of vulnerability in later life. Nature Human Behaviour, 2, 551–558. https://doi.org/10.1038/s41562-018-0395-3
Ihle, A., Ghisletta, P., Ballhausen, N., Fagot, D., Vallet, F., Baeriswyl, M., Sauter, J., Oris, M., Maurer, J., & Kliegel, M. (2018). The role of cognitive reserve accumulated in midlife for the relation between chronic diseases and cognitive decline in old age: A longitudinal follow-up across six years. Neuropsychologia, 121, 37–46. https://doi.org/10.1016/j.neuropsychologia.2018.10.013
Spini, D., Bernardi, L., & Oris, M. (2017). Toward a life course framework for studying vulnerability. Research in Human Development, 14(1), 5–25. https://doi.org/10.1080/15427609.2016.1268892
Stern, Y. (2009). Cognitive reserve. Neuropsychologia, 47(10), 2015–2028. https://doi.org/10.1016/j.neuropsychologia.2009.03.004