Kalisch et al. make a convincing case that a positive appraisal style is the proximal cause of transdiagnostic resilience. Another relevant question is, what causes positive appraisal? In other words, what is the distal cause of resilience? In my commentary, I argue that early experiences determine an individual's appraisal of a certain situation and are therefore a distal, or ultimate, cause of resilience.

Animals, including humans, have been shown to have considerable phenotypic plasticity. That is, individuals are able to adjust their phenotype to meet environmental demands. This phenomenon has been extensively studied in developmental contexts. During early childhood, including the fetal period, individuals are especially sensitive to programming effects of their environment (e.g., Gluckman et al. Reference Gluckman, Hanson and Beedle2007; Lupien et al. Reference Lupien, McEwen, Gunnar and Heim2009).

Individuals sample their environment for cues that more or less reliably predict their future environment (Kuzawa Reference Kuzawa2005). Through these cues, they learn about their environment and gradually adjust their phenotype to match specific demands (Frankenhuis & Panchanathan Reference Frankenhuis and Panchanathan2011). For example, humans are hypothesized to adjust their stress-responsivity, which is closely tied to their appraisal of stressful situations (Del Giudice et al. Reference Del Giudice, Ellis and Shirtcliff2011; Ellis & Del Giudice Reference Ellis and Del Giudice2014). Although phenotypic programming might result in resilience in environments that match the phenotype adjustment, it is known also to have an important trade-off. If the later environment turns out differently than predicted, a phenotype-environment mismatch can have detrimental effects (Frankenhuis & Del Giudice Reference Frankenhuis and Del Giudice2012; Nederhof & Schmidt Reference Nederhof and Schmidt2012).

Early in life, individuals are most likely to adapt their phenotype to meet future environmental demands, compared with any other period in life. Traditionally, this has been seen as a window of vulnerability. More recently, researchers have started to appreciate the possible positive implications of such a period of increased sensitivity (Andersen Reference Andersen2003). The window of vulnerability has become a window of opportunity. During this window of opportunity, individuals are most sensitive to both positive and negative cues in their environment, either to profit optimally from the environment or to arm oneself optimally against it (Boyce & Ellis Reference Boyce and Ellis2005; Del Giudice et al. Reference Del Giudice, Ellis and Shirtcliff2011).

Several animal studies also support the notion of resilience in matched and vulnerability in mismatched situations. For example, Champagne et al. (Reference Champagne, Bagot, van Hasselt, Ramakers, Meaney, de Kloet, Joëls and Krugers2008) showed in rats that offspring of low-licking and -grooming dams performed better in a contextual fear-conditioning paradigm compared with offspring of high-licking and -grooming dams. Variation in licking and grooming behavior occurs naturally and can be influenced by stressfulness of the environment (Ivy et al. Reference Ivy, Brunson, Sandman and Baram2008). Other studies showed similar effects (Daskalakis et al. Reference Daskalakis, Oitzl, Schachinger, Champagne and de Kloet2012), also when using other stress-paradigms, such as maternal separation (Biggio et al. Reference Biggio, Pisu, Garau, Boero, Locci, Mostallino, Olla, Utzeri and Serra2014; Zalosnik et al. Reference Zalosnik, Pollano, Trujillo, Suarez and Durando2014), limited nesting material (Santarelli et al. Reference Santarelli, Lesuis, Wang, Wagner, Hartmann, Labermaier, Scharf, Müller, Holsboer and Schmidt2014), or prenatal stress (Van den Hove et al. Reference Van den Hove, Leibold, Strackx, Martinez-Claros, Lesch, Steinbusch, Schruers and Prickaerts2014). It must be noted, however, that not all animal studies support the mismatch hypothesis (see Nederhof & Schmidt Reference Nederhof and Schmidt2012).

This might point in the direction of adversity having adaptive programming effects in some but damaging effects in others. Nederhof and Schmidt (Reference Nederhof and Schmidt2012) proposed that programming sensitivity might differ between individuals. This suggestion was based on the observation that natural selection favors diversity, because the future is uncertain. Indeed, it has been suggested that, from an evolutionary perspective, it makes sense to invest in offspring that differ in sensitivity to programming effects (Belsky et al. Reference Belsky, Bakermans-Kranenburg and van IJzendoorn2007; Ellis et al. Reference Ellis, Boyce, Belsky, Bakermans-Kranenburg and van Ijzendoorn2011). Individuals with high programming sensitivity are hypothesized to adjust their phenotype, including appraisal, in a way that is expected to be adaptive in their later environment (Nederhof & Schmidt Reference Nederhof and Schmidt2012). As a consequence, appraisal styles are expected to be more positive in matched compared with mismatched situations in these individuals, resulting in resilience in matched situations and vulnerability in mismatched ones. In individuals with low programming sensitivity, stressful events are hypothesized to accumulate and increase the risk for stress-related diseases (Nederhof & Schmidt Reference Nederhof and Schmidt2012).

Recent evidence prudently supports this notion. Oldehinkel et al. (Reference Oldehinkel, Ormel, Verhulst and Nederhof2014) showed that early adversity increased the risk for early-onset depression, but not for later onsets. They also showed that those who experienced adversity but were not affected by depression before the age of 16 seemed more resilient under stressful conditions compared with others who did not experience early adversity. A similar finding was published by Nederhof et al. (Reference Nederhof, Ormel and Oldehinkel2014), who showed that the interactive effects of early and recent adversity depended on attention style, assuming that attention style could be the result of programming effects. It must be noted, however, that these two studies were conducted in the same sample.

As far as I am aware, there is one other study in humans providing evidence for resilience in matched and vulnerability in mismatched situations. Brody and colleagues (Reference Brody, Yu, Chen, Miller, Kogan and Beach2013) showed that African-American youth from high-risk backgrounds with high self-control and social/academic competence had more physiological risk factors than did youth with similar backgrounds but low self-control and competence. On the other hand, these high-risk/high-self-control youth showed fewer depressive symptoms compared with high-risk/low-self-control youth.

Unfortunately, in none of these studies was appraisal style used. Future research should investigate whether appraisal style is affected by early adversity in an adaptive way that could constitute resilience later in life. If such a notion would be supported, we would come closer to an answer to the question of what the distal cause for resilience might be.