Across 195 countries, rates of depressive disorders in women & men are higher among islanders (relative to mainlanders) at more northern locations in the Northern Hemisphere & at more southern locations in the South'n Hemisph

Who is more prone to depression at higher latitudes? Islanders or mainlanders? Van de Vliert, Evert; Rentfrow, Peter Jason. Current Research in Ecological and Social Psychology, 2(1), [100012]. Jul 2021. https://doi.org/10.1016/j.cresp.2021.100012

Abstract: Across 195 countries, rates of depressive disorders in women and men are higher among islanders (relative to mainlanders) at more northern locations in the Northern Hemisphere and at more southern locations in the Southern Hemisphere. Our explanatory analyses show that the three-way interaction of greater daylength variability, being more of an islander, and adopting a more individualistic culture accounts for higher rates of depression in both genders. Differences in longitude, photoperiod, phase shift, disaster risk, economic poverty, income inequality, and urbanization level do not appear to account for the oppositely sloping north-south gradients of depression above and below the equator.

6. Discussion

Nowhere in textbooks do we learn that the world’s mental depression rates have a north-south rather than east-west distribution. Nor are we informed about the existence of a U-curve distribution of depression between the north and south poles. The present results help fill these gaps in our knowledge, highlighting the remarkable fact that the equator-topole increases in depression in the Northern and Southern Hemisphere are steeper for islanders than mainlanders. This tends to hold even for men, who have always had, and continue to have, lower rates of major depression than women in all countries on Earth (cf. Andrade et al., 2003; Mersch et al., 1999; Weissman et al., 1996). The two distinct generalizations—across hemispheres and genders—lend considerable credibility to the conclusion that islanders relative to mainlanders are more prone to depression at higher latitudes. This conclusion could be further strengthened by within-country replications. Biogeographers will not be surprised by this finding as they have known for a long time about the special relationships between islands and illnesses (Harcourt, 2012, 2015). They speak of island rules to describe and explain how plants, animals, and humans on islands differ biologically from plants, animals, and humans on the mainland. One such rule is that infectious diseases in humans have flatter latitudinal gradients across island locations with sea climates than across mainland locations with continental climates (Cashdan, 2014). The new island rule developed here can be summarized as steeper equator-to-pole increases in rates of depression among islanders than among mainlanders. While it is hard to come up with an explanation for this geography of depression, there are at least five reasons to believe in the existence of a worldwide ecology of depression modulated by habitat differences in daylength and islandness. First, in common with the variability in daylength—and thus in photoperiod, phase shift, seasonal temperatures, daily rainfall, plant growth, and animal life—rates of depression fail to vary along the Earth’s east-west axis, pointing to a latitude- rather than longituderelated explanation of depression. Second, among islanders, and especially so among individualistic female islanders, increases in daylength variability tend to go hand in hand with increases in rates of depression. Third, among mainlanders, and especially so among collectivistic mainlanders, increases in daylength variability tend to go hand in hand with decreases instead of increases in rates of depression. Fourth, photoperiod (Lingjaerde et al., 1986; Potkin et al., 1986; Young et al., 1997) and phase shift (Avery et al., 1997; Lewy et al., 1984; Rosenthal and Wehr, 1992) cannot account for the oppositely sloping latitudinal gradients of depression among islanders and mainlanders. Finally, people who live at higher latitudes or on islands and clear peninsulas tend to have more individualistic cultures that make them more prone to depression (cf. Draguns, 1997; Hofstede, 2001; TanakaMatsumi and Draguns, 1997). Our reading of this finding is that individualistic cultures serve as double-edged swords. Daylength variability and islandness regularly disturb life and activities so that control has to be restored by using internal agency and creating internal structure (Friesen et al., 2014; Kay and Eibach, 2013; Whitson and Galinsky, 2008). On the one hand, these active control-restoring strategies in individualistic cultures leave the large majority of higher-latitude inhabitants and islanders more creative and happier in consequence (cf. Van de Vliert and Van Lange, 2019). On the other hand, one might speculate that a small minority of higher-latitude inhabitants and islanders may passively fall back on adopting overly simple and stable interpretations of the ordered environment that are unrelated to the controlreducing events (e.g., believing society is systematically divided into haves and have-nots; Landau et al., 2015). Future research may seek to show that this minority tends to use maladaptive control-restoring strategies associated with depressive feelings of hopelessness and helplessness rather than elated feelings of happiness. These explanatory considerations should be read in the knowledge that the geography and ecology of depression cannot be convincingly studied in a controlled laboratory setting. Thus, cause and effect cannot be inferred, so that further investigations are needed to exclude rival explanations in terms of, for example, the greater exposure of higherlatitude inhabitants to insufficient ultraviolet radiation and cold stress. Mitigating this drawback is the almost axiomatic assumption that rates of depression in women and men cannot have caused widely different degrees of daylength variability and territorial water borders. Hence, there is a causal quality to our robust relationship: the only possible direction of impact is from daylength variability and islandness to depression. The same latitude-by-longitude design with nested islandness can be employed to explore the geography and ecology of, for example, treatment for depression (Smits and Huijts, 2015), control of neglected tropical diseases (Garchitorena et al., 2017), and disparities between islanders and mainlanders in healthy behaviors such as consumption of fruits/vegetables and abstinence of tobacco/alcohol use (Taylor et al., 2018). That causation cannot be firmly established is not the only shortcoming of the proposed explanation of islanders’ and mainlanders’ rates of depression at higher latitudes. Additionally, the proposed mediating mechanism of the maladaptive control-restoring strategy of adopting overly simple and stable interpretations of the ordered environment (Landau et al., 2015) was not measured and analyzed. Mitigating this weakness is the complicating fact that a viable alternative theory should explain why the higher rates of depression at higher latitudes hold for islanders more than for mainlanders (Fig. 1). No alternative theory that we know of does so. The potentially confounding effects of disaster risk, economic poverty, income inequality, and urbanization level have been ruled out. As a result, this article provides novel evidence that brings global geography and ecology together to better understand why some populations are more depressive than others. Given the importance of depression for healthy human functioning, this is no small gap to fill.