Delayed sleep onsets may be due to a phase-delay of the circadian pacemaker controlling sleep onset, an acute effect of evening light on alertness or both. Additional studies designed to measure circadian phase physiologically are required to address this. In either case, the results support a hypothesis that exposure to artificial light after sunset can delay sleep onset and reduce sleep duration. Such effects can be expected to increase as access to electricity on Tanna Island expands. Historical writings have been taken to suggest that in pre-industrial western European populations, nocturnal sleep habitually occurred in two bouts – so-called ‘first and second sleep’ – separated by an hour or more of midnight waking. Segmented patterns suggestive of ‘first and second sleep’ have been observed in a small-scale agricultural society in Madagascar. In the present study, 14% of the 519 recorded nights exhibited a bout of nocturnal waking that was sufficient for the state detection algorithm to score two separate bouts of sleep. However, this pattern does not appear to ft with the concept of ‘first and second sleep’ as a reliable sleep phenotype. About 50% of the subjects showed evidence of split sleep, and among those that did, the pattern was sporadic, with split sleep typically evident on only one of up to eight nights of recording. Splits also occurred at variable times of night. Nocturnal sleep in the indigenous residents of Tanna Island is therefore best described as monophasic, with occasional opportunistic daytime naps. An increase in the number of sleepers sharing the same sleep surface or room can reduce sleep efficiency or duration. Te number of co-sleepers was slightly greater in the non-electric communities,raspberry container size but sleep duration and efficiency were lower in the electric communities. Also, there was no association between the number of co-sleepers and sleep duration and efficiency in either community.
In addition, residents of Tanna commonly sleep on hard surfaces, which would not transfer movement between adjacent co-sleepers. For these reasons, it is unlikely that any differences in the number of co-sleepers would account for differences in sleep duration and efficiency between communities.Although the range of ages was limited in the present sample , a negative relationship with age and sleep efficiency did emerge, but only in non-electric communities. In communities with an electric grid, the lack of a relationship indicated that young adults also exhibited lower sleep efficiency, something that was not apparent in the young adults of the non-electric communities. A non-electric Haitian population showing lower sleep efficiency in younger adults suggests lifestyle factors, such as, increased childcare responsibilities, nocturnal household duties, or engagement in social activities that mask physiological age differences, all of which may be more easily facilitated in Tanna’s electric communities. On Tanna Island, males are often up late drinking kava, which is an important custom in Vanuatu, as the nakamal, where kava is consumed, is an important gathering place for older men to pass along knowledge and advice to young males in the village. Kava is mildly sedating, but males and non-breastfeeding females pooled separately across communities did not exhibit difference in sleep duration or efficiency, suggesting minimal effect of Kava consumption on sleep. We do not have sufficient information to be able to separate and compare sleep on nights with and without Kava consumption, and any effects might not be detectable by actigraphy. A limitation of this study is that data were collected only in April and May, during the transition from summer to winter, when daylength averages ~11.5 h. Seasonal variation in sleep timing has been reported in traditional hunter-gatherer societies, with daily wake-up time in one study population tracking seasonal changes in the time of the daily minimum of environmental temperature more closely than changes in the time of sunrise. In the present study, temperature recorded in representative sleeping huts showed a daily minimum that occurred on average 26min after sunrise, in both the coastal and the inland villages. Wake onsets on average were closer to sunrise than to the ambient temperature minimum in both groups. Ambient temperature may be less significant for sleep timing on Tanna Island because nights are milder, the daily temperature range is modest , and the transition from decreasing to increasing temperature is gradual.
Also, temperature is mild throughout the year and seasonal variation is modest. In this respect, the natural sleep environment on Tanna Island may be more similar to modern built sleep environments, in which temperature changes from day to night are minimized. If ambient temperature plays a role in sleep timing, then we would not expect to see a large effect of seasonal temperature changes on sleep parameters in residents of Tanna Island. The results of this study indicate that sleep measured by actigraphy in the small-scale traditional society on Tanna Island can be differentiated from sleep in western industrialized samples by relatively long duration and low efficiency. Availability of on-demand electric light appears to have a detectable effect on nocturnal sleep onset and duration, but this effect is likely mitigated by exposure to natural light throughout the day. Actigraphy studies of indigenous Ni-Vanuatu living in industrialized population centers elsewhere in Vanuatu may provide further insight into how lifestyle and industrialization shape sleep.Decades of both theoretical and empirical research based on the polygyny threshold model have suggested that polygyny should be more common and more pronounced in populations in which males differ substantially in resource control. In humans, this will be in socio-cultural contexts where wealth is held predominately by men, and where there is high inequality in its distribution. Historical and cross-cultural records, however, suggest that polygyny became less common as relatively egalitarian horticultural production systems transitioned into agricultural production systems, in spite of the fact that agriculture is characterized by both a greater importance of material wealth in the production process and greater levels of material wealth inequality than horticulture. This is the polygyny paradox. Existing hypotheses for the rise of monogamy with historic agricultural populations invoke the increasing importance of rival1 material wealth among agriculturalists, inheritance rules in conjunction with paternity certainty, male power relations, declines in female contributions to production, pathogen risk and punishment and cultural group selection via the imposition of norms.
Since human behavioural variation is often determined by many underlying factors, there are likely to be complementary effects among the potential causes identified in these hypotheses. Specifically, there should be coevolutionary interactions between the individual-level, economic- and fitness-based explanations for the rise of monogamy advanced here, and the cultural evolutionary explanations provided by Henrich et al. and Bauch & McElreath. Our results show how individual fitness maximization can explain the de novo origins of predominant monogamy within highly unequal populations. Should monogamy have group-level fitness benefits as suggested by Henrich et al., its emergence in specific groups via the mechanism we propose would provide the source populations for cultural group selection dynamics to propagate monogamy to other populations. Explanations for the rise of monogamy in agricultural societies in the spirit of Alexander and Henrich et al. develop the idea that powerful leaders might have imposed monogamy on the masses because such a marriage norm leads to greater in-group male–male cooperation,raspberry plant container improving the success of the group in inter-group contests. The economically grounded explanation for the rise of monogamy that we present here is not necessarily in competition with such theories. Our model, however, establishes that basic changes in the structuring of wealth inequality coinciding with the rise of class-based societies would have made monogamy adaptive at the individual level in a large fraction of the population—greatly increasing the scope for hypotheses advancing hierarchical imposition or even frequency dependent social transmission of norms for monogamy. The present analysis builds on work recognizing the importance of inherited wealth in structuring family relationships. To this existing literature, we introduce a new individual-level, cross-cultural dataset of wealth, marriage and reproductive outcomes, numbering 11 813 records from 29 human populations, including hunter –gathers, horticulturalists, agropastoralists and agriculturalists. Our dataset is unusual in both its scope and in the availability of individual-level information, rather than qualitative societal summaries. While not without its limitations—discussed in more detail throughout—it captures the core features of the polygyny paradox. Following Oh et al. , we develop a model of the equilibrium fraction of women married polygynously in a population where the extent of polygyny is determined by the fitness maximizing choices of both men and women. In contrast to the standard polygyny threshold model, which is a one-sided mate choice model that allows only for female choice, we develop a mutual or two-sided model. In this model, male choice refers not to selecting particular females on the basis of their quality , but rather to the male’s choice of the number of wives that will maximize his fitness. A male’s demand for wives depends on his level of wealth and the costs of mating investment, and can be more than, less than or equal to the total number of women who would choose to marry him. Mutual mate choice is rare in nature, but the conditions for it are met in species in which biparental care is important for the survival of offspring, as is typically true of humans.
From our theoretical model, we identify two conditions that jointly can lead to a decrease in the population-level frequency of polygyny in highly unequal agricultural populations: in these highly stratified economies, the fraction of men with sufficient wealth to make polygynous marriage an attractive option for them and their potential partners is low relative to other subsistence systems, and decreasing marginal fitness returns to increasing number of wives above and beyond the fitness costs of sharing a husband’s wealth sharply limit the number of wives acquired by exceptionally wealthy individuals. We use our empirical data to demonstrate that the transition to agriculture is associated with both of these factors identified as drivers of monogamy.The Standard Cross-Cultural Sample illustrates that the frequency of polygyny is relatively high in horticultural and pastoral populations, and low in agricultural populations. These findings are robust to use of quantitative or qualitative descriptors of polygyny. The third panel presents our estimates of the extent of material wealth inequality among males in the four production systems. Theoretical models of mating systems predict that polygyny should be positively associated with inequality in male resources, and more specifically with what Murdock terms ‘movable property or wealth which can be accumulated in quantity by men’. These forms of rival material wealth are, as we have just seen, more unequally held in horticultural economies than among foragers, which is consistent with the greater extent of polygyny in the former.Oh et al. show that inequality in reproductively important, non-rival forms of wealth—network ties, genes conferring adaptive phenotypes or acquired knowledge, for example— can also be a strong driver of polygyny, contributing to the explanation of substantial levels of polygyny in some societies with little rival wealth inequality. Indeed, there is empirical evidence that non-rival forms of wealth are associated with polygynyous marriage in some foraging and horticultural populations. While the polygyny threshold model has been effective in predicting the distribution of polygynous males within populations , the reduced level of polygyny in agricultural populations typically characterized by greater inequality poses a serious challenge to existing models of mating and marriage.To address this challenge, we build a comparative database of individual-level wealth, marriage and reproductive success measures in 29 diverse populations distributed over a wide geographical range . Table 1 provides population-specific background data. In order to use all cohorts of the adult male populations, relevant measures—wives and wealth proxies—are age adjusted in a Bayesian framework to represent their predicted values at age 60. This method of age adjustment assumes that the additional acquisition of wives and wealth from the time of censor to the age of 60 are unmeasured positive random variables, with mean values governed by the remaining time for acquisition and the age-specific acquisition rate trajectories inferred from the population cross sections . Our polygyny measures reflect the per cent of women who will ever be married to a man who marries more than once—in other words, in contrast to the data in figure 1a, we consider sequential marriage as a form of polygyny since the offspring of each mother are rival claimants to a father’s property.