The impact of stress-related mental health diseases in people living in urbanized cities has been widely recognized [1–4]. The rapid urbanization process has persuaded many people living in rural communities to move into cities, since urbanization entails several advantages over rural areas, for example, better employment opportunities, higher incomes, better nutrition and education, primary health services, and more entertainment options [2]. However, one of the main outcomes of this process is the exposition to many stressors, such as crowding, traffic, more working hours, pollution, noise, and less green spaces, which negatively impact mental and physical health of people [1, 3, 5]. Accordingly, previous literature has consistently reported that living in urbanized environments is a risk factor for the development of some stress-related psychiatric disorders [4, 6]. For instance, some studies have documented a higher prevalence of major depressive disorder, schizophrenia, bipolar disorder, and non-affective psychosis in people living in urbanized cities [6–8]. Besides, it has been reported an increased brain amygdala activity associated with social stress processing in city dwellers compared to town or rural dwellers [9]. However, differences in mental health of people living in urban and rural areas, among high- middle-, and low-income countries have not been consistently described. Most of rural citizens in low- and middle-income countries are still engaged in agricultural activities, without the technological developments to facilitate production, coupled with low levels of access to comprehensive health and education coverage; thus, rural families often live in constant financial uncertainty and poverty; these low socioeconomic conditions have been commonly linked to social stressors such as violence and crime scenes [10].
On the other hand, it has been widely documented that experiencing traumatic and stressful life events can result in altered reactivity of the stress response system, the hypothalamic–pituitary–adrenal (HPA) axis [11–13]. Together with limbic structures such as the amygdala and hippocampus, the HPA axis is required for the expression of appropriate stress responses. The first pathway of stress response is driven by activation of corticotrophin releasing hormone (CRH) from the hypothalamic paraventricular nucleus (PVN), which results in the release of adrenocorticotropic hormone (ACTH) from the hypophysis, and finally in glucocorticoids (GCs) secretion (corticosteroids or cortisol in humans) from the adrenal glands [12, 14]. All this process temporarily interrupts the metabolic homeostasis of the body; however, GCs also act in peripheric organs such as the liver to mobilize energy reserves, i.e., glucose, to maintain the body homeostasis and adaption to environmental demands [15]. Once the stressor is over, GCs activate the negative feedback mechanism by inhibiting further HPA axis activity; alterations in this regulation system might provoke abnormally high or low GCs secretion [16]. It is known that individuals might habituate to chronic psychosocial stressors decreasing the HPA axis activation to promote adaption to challenge and to protect the body for the negative effects of prolonged GCs secretion [17]. Another steroid hormone secreted during challenging situations is testosterone [18–19], a reproductive hormone synthesized in high concentrations in males and lower concentrations in females, as well as in the adrenal glands in both sexes [20]. The nature of testosterone response varies according to personality traits and contexts, e.g., whether the situation represents a social threat, a stressor, or an anticipation of a challenge [18, 21]. For instance, Bedgood et al. [21] reported that young adult men with lower basal cortisol levels had higher increases in testosterone levels after a social evaluative stressor, which might confer more competitive skills. Although there is evidence indicating a heightened vulnerability to mental problems as well as a higher stress response in people raised in cities than in small towns or rural areas, this information mainly comes from high-income countries (e.g., Germany [9, 22], the Netherlands [2, 23], U.S. [24], Denmark [5], UK [4]), restricting its generalizability to other populations. To the best of our knowledge, studies on alterations in the HPA axis reactivity and their association with mental health in low- and middle-income countries are very scarce, where non-urban citizens may be affected by stressors such as poverty and violence.
In this regard, Mexico is a middle-income country [25] characterized by being a heterogeneous country, both in population and territory, as it has faced an economic growing progress with regional disparities and inequalities. In contrast to the rural and suburban (i.e., in process of urbanization) regions of the country, the big urban cities, such as Mexico City, the capital of the country (an urban city with around 9,209.944 million inhabitants) [26], has greater access to resources and services, e.g., health care system, educational attainment, more entertainment options, and better jobs [27]. However, urban citizens might live in scenarios where insecurity and violence are common without advertising the magnitude of the problem because of the population density. For instance, urban areas are so crowded and complex that their inhabitants are not always aware of the different forms of violence, direct (being a victim) or indirect (e.g., witnessing, watching, or hearing the news), occurring in a single day (e.g., assaults, homicides, or femicides). Contrastingly, in rural and suburban areas, the levels of violence (mostly direct) vary from those in urban areas, but the lower population density increases the perception of insecurity [28]. Witnessing a crime can be distressing, particularly to those people who has also been a victim of a crime (mild or severe) [29]. In fact, previous literature has reported that mental health consequences for victims of direct and indirect violence are the same [30].
Unfortunately, the insecurity issues exceed the authority’s capability to adequately deal with them [31]. Then, the insecurity perception and the uneven distribution of progress that face rural/suburban citizens in marginalized areas, contribute to a reduced self-perception of well-being. In contrast to the Capital of the Country, other cities such as the State of Mexico (a geographical region adjacent to the capital) is one of the most violent areas in the country with the highest national crime incidence including kidnappings, extortion, drug dealers, and the highest rate of feminicides in the Country [26, 31]. Despite being one of the most populated areas in the Country, the State of Mexico has many contrasting small suburban communities (whose main economic activities are manufacturing, mining, and agricultural industry), some of them with less than 50,000 inhabitants and others with less than 5,000 inhabitants. Compared to Mexico City, people living in those communities have little access to basic services such as public transportation or specialized health care services, which sometimes are far away from their places of residence [32, 33]. Therefore, the goal of the present study was to compare cortisol and testosterone secretion in response to images with violent and traumatic content as well as symptoms of psychopathology and psychological distress between people living in an urban big city and suburban communities. Considering that prolonged exposure to violent scenarios could create an emotional desensitization process [34], we hypothesized that people living in the suburban communities will have a reduced cortisol response (i.e., blunted, or flattened reactivity) reflecting a process of habituation and an increased number of distress symptoms signaling poorer mental health compared to people living in the urban city (with better security perception). Besides, since individual differences in stress reactivity might depend on how people appraise adversity -as challenges or social threats- we expected an inverse relationship between testosterone and cortisol secretion.