INTRODUCTION
The COVID‐19 pandemic brought the world to a halt. Since early 2020, social life has changed for many people around the world. Government restrictions and new social norms led to a reduction in mobility (Google, 2020), avoidance of public transport, cancellation of the majority of large events such as concerts, festivals, religious and sports events, and the temporary closing down of meeting places such as cafes, restaurants, museums or theatres. The COVID‐19 outbreak and measures undertaken by almost all the countries around the world pose numerous threats to people's psychological well‐being; thus, we believe that focusing on our closest social environment is greatly needed. Both relative lack of social relations (Tay et al., 2013) and negative emotions (Huppert, 2009) strongly predict overall mortality and disease outcomes, and the threat of SARS‐CoV‐2 may increase anxiety levels (Karwowski et al., 2020) and prejudice toward other nationalities (Sorokowski et al., 2020), rendering any effort to understand stress in isolated or quarantined individuals during the COVID‐19 pandemic even more important. Moreover, to understand possible negative effects, it is important to unpick the pre‐existing factors that can predict the stress levels of people in quarantine or isolation.

Other People
One such factor is the presence (or absence) of other people. To date, most of the existing studies on human isolation and confinement are naturalistic observational studies, such as the SEALAB project (Radloff & Helmreich, 1968), or the South African National Research station (SANAE) in Antarctica (Vermeulen, 1977). Under these difficult circumstances of a physically and socially restricted environment, participants reported high indices of severe hostility, depression, insomnia, and anxiety. Studies on prisoner populations suggest that social factors can significantly boost prisoner’s well‐being, which can otherwise be disrupted by jail isolation (Kyprianides & Easterbrook, 2020). The conditions in which participants of the aforementioned studies were put are far from what people are experiencing now. Although being relatively isolated from the outside world, many of us remain within the comfort of our own homes, usually surrounded by friends or relatives. However, there are also people who are getting through this time alone. Living alone has been previously linked to higher indices of depression and anxiety, and other common mental disorders (Jacob et al., 2019). Therefore, those who live alone might experience more stress due to the COVID‐19 situation than those living with people who may be a source of potential support (Cohen & Wills, 1985). On the other hand, being around others might be stress‐enhancing. Specifically, the company of others can be detrimental to mental health when one is subjected to overcrowding—or, in the current situation—residential or household crowding, highly prevalent in developing countries (Epstein, 1981). Studies suggest that this type of chronic stress is often accompanied by a lack of privacy, a higher number of unwanted social interactions, possible deterioration of relationships with the family or flatmates, and so forth (Fuller et al., 1996). Considering the above, we hypothesise that the relationship between the number of people one is stuck with during isolation is somewhat U‐shaped—those who live alone and those who are subjected to overcrowding experience the highest levels of stress.

Intimate Relationships
In general, married individuals are happier (Lee & Ono, 2012), live longer and healthier lives (Kiecolt‐Glaser & Newton, 2001), and are at a lower risk of committing suicide (Waite & Gallagher, 2001). A recent study by Chin et al. (2017) supported previous conclusions at a physiological level. Interestingly, the authors investigated the cortisol levels (a hormone that is often associated with stress) of married and single persons, and found that married individuals had lower cortisol levels than unmarried and previously married people, which would suggest that they experience lower levels of stress. Similar patterns have been observed among persons in intimate relationships (not necessarily married), who exhibited lower levels of stress compared with single persons when stress was assessed by blood pressure (Sisca, 1985), self‐reported measures (Hudson & O'Regan, 1994), or a broad range of well‐being indices (Sorokowski et al., 2019).
Despite numerous advantages, being in a romantic relationship also carries a number of risks, for instance, contagion of negative emotions (Roberts & Levenson, 2001). Moreover, when couples encounter difficulties, such as financial hardship, or a lack of support from the spouse, they may experience a pronounced decrease in marital satisfaction, which can lead to an increase in stress levels (Archuleta et al., 2011).
On the other hand, intimate relationships allow for dyadic coping (for a review, see Bodenmann, 2005), and often serve as a buffer against difficult situations (Gottlieb & Wagner, 1991), which may in turn translate into lower levels of stress (Chin et al., 2017). Here, we intend to investigate whether marital status is a significant predictor of stress levels during quarantine or isolation on a large dataset from numerous countries around the world. We hypothesise that during the difficult times of relative isolation, being in an intimate relationship (i.e. marriage or cohabitation) is more beneficial in terms of experienced stress levels than being single.

Number of Children
Also of interest is how having and living with children relates to the experiences of adults during the period of relative isolation. Having children is quite a positive experience, as it has been generally related to greater life satisfaction, especially amongst married couples in contrast to individuals who are separated, cohabiting or never‐married singles (Angeles, 2010). Nevertheless, having children may also foster elevated stress levels. Parental stress is the result of the gap between the challenges of being a parent and the parent’s perceived ability to cope with those challenges (Abidin, 1995). This kind of stress may be even more pronounced during the COVID‐19 outbreak, as most parents now must homeschool their children. Parental stress has been associated with numerous negative feelings and thoughts, such as feeling overwhelmed and dissatisfied, and also perceiving the child as difficult (Haskett et al., 2006). For instance, a study on parents from 94 countries showed that parenthood is negatively linked to well‐being (Stanca, 2012).
As outbreaks make individuals vulnerable to depression and diminished social support (Stein et al., 2005), the current COVID‐19 pandemic may compromise parenting and childcare practices. Concerns over a child younger than 16 years getting COVID‐19 have been reported as very common among parents in China (Wang et al., 2020). During the equine influenza outbreak in Australia, Taylor et al. (2008) found that individuals having one child had a 1.2 times higher risk of experiencing distress than those with no children. Therefore, we hypothesise that having children at home may be associated with higher levels of stress in adults.

Gender
Gender seems to be related to well‐being (Mroczek & Kolarz, 1998) and stress levels (Taylor et al., 2008). Previous studies have found that women report greater sadness, anxiety, and stress than men (Bergdahl & Bergdahl, 2002; Gao et al, 2019).
Concerning quarantine, evidence from the first outbreak of equine influenza in Australia showed no gender differences on the level of psychological distress (Taylor et al., 2008). However, a recent study by Limcaoco et al. (2020) in 25 countries on susceptibility to stress during the COVID‐19 situation indicated that women report greater levels of stress. Similar gender differences for stress, anxiety, and depression symptoms were found by Wang et al. (2020) in a Chinese sample during the initial stage of the COVID‐19 outbreak, although only a minority of the participants reported being confined. In sum, while the evidence suggests that women in normal circumstances experience more stress, support for the link between gender and stress under quarantine is inconclusive.

Age
Generally, stress levels tend to decrease with age and although older adults report poorer perceived health, they have lower stress levels and higher well‐being than young adults (Archer et al., 2015). Bergdahl and Bergdahl (2002) found that self‐reported stress levels increase from the age of 20 to a peak in the 40s, and then decrease to the lowest level in the 60s. Stone et al. (2010) argued that well‐being increases after 50 years of age, proposing an inverted U‐shaped relationship between age and well‐being (but see Frackowiak et al., 2020). Studies generally support the notion that older people are less affected by stressors than younger people (Feizi et al., 2012).
Few studies have investigated the relationship between age and stress levels during quarantine. The objective consequences of being infected by SARS‐CoV‐2 are higher for the older population (World Health Organization, 2020a, 2020b). However, while the perceptions of a highly transmitted illness threat among the elderly may prompt stress, the empirical evidence is mixed. For instance, Taylor et al. (2008) found that the negative psychological impact of quarantine is more pronounced among younger people. Investigating psychosocial responses towards a national outbreak of SARS in Singapore, Sim et al. (2010) found that psychiatric morbidities are associated with younger age and higher posttraumatic stress symptoms. Most recently, an online survey in the early stages of the COVID‐19 outbreak in China found no association between age and stress (Wang et al., 2020).
As such, although older adults face higher risks of severe disease and death due to COVID‐19 (WHO, 2020b), the evidence suggests that older people are less stressed and less affected by psychological consequences of quarantine and social isolation, while younger ones exhibit the highest levels of stress.

Individualism–Collectivism Dimension
The role of culture has been widely studied for decades, with researchers debating how cultural factors may act as a buffer to the environmental stressors or, on the contrary, exacerbate stress levels (Dar, 2017). One of the main focuses of research in this area, the individualism–collectivism dimension (Hofstede, 2001), has been linked to stress processing (Chun et al., 2006). During the current quarantine, people have been forced to renounce their personal enjoyment (e.g. sports, concerts, shopping, travel, social gatherings) for the sake of group needs. Since individualistic, rather than collectivistic, cultures put a higher value on pleasure and hedonism (Schwartz, 2009), it might be reasonable to think that the emotional cost of this quarantine period would be greater in individualistic cultures. In fact, collectivistic (vs. individualistic) cultures put more emphasis on group harmony over personal interests and enjoyment (Triandis et al., 1990). Moreover, Oarga et al. (2015) found that helping behaviours had a stronger association with life satisfaction in countries where helping others constituted a social norm. In this sense, people from collective cultures would be likely to focus on caring for others, thereby alleviating the negative psychological consequences of quarantine. Overall, under the prevailing quarantine, the level of stress is expected to be higher for individualistic cultures compared to the collectivistic ones.

Aims and Hypotheses
The present study is one of the first to test how the number of persons an individual is staying with in isolation, along with age, gender, marital status, the number of children, and culture (i.e. individualism vs. collectivism) is associated with experienced stress levels in a large sample from 27 countries and areas. Furthermore, we want to investigate the link between stress levels and educational background and the severity of the COVID‐19 situation in each country or area. Based on the literature review, our pre‐registered hypotheses (see https://osf.io/xf4mj) were as follows:
H1: Individuals living alone and those who are subjected to overcrowding while in isolation experience the highest levels of perceived stress (compared with individuals living with others).
H2: Married and cohabiting persons experience lower levels of stress compared with persons that are single.
H3: Individuals with children would report increased stress levels during the COVID‐19 pandemic compared to people living alone or with adults. Levels of perceived stress would increase as the number of children at home increases.
H4: Younger people experience more stress than older people.
H5: People in collectivistic cultures experience less stress than those in individualistic cultures.
H6: Women experience higher levels of stress compared with men (not pre‐registered hypothesis).