Fincher & Thornhill (F&T) have presented cross-national and American data to support the hypothesis that pathogen stress is positively associated with family ties and religious affiliation constituting in-group sociality. Their evolutionary argument that pathogen stress is an ultimate cause of family and religious sociality is well grounded both in theory and in data. However, their broad-stroke analyses and explanations leave out two seemingly counterintuitive conclusions which we try to address by attending to more detailed analyses within the same pathogen-stress framework.
First, with respect to the hypothesis about pathogen-driven family ties, existing research based on the Standard Cross Culture Sample (Murdock & White Reference Murdock and White1969) shows a positive association between pathogen stress and polygyny (Katz & Konner Reference Katz, Konner and Lamb1981; Low Reference Low, Betzig, Borgerhoff Mulder and Turke1988; Reference Low1990; Marlowe Reference Marlowe2000; Reference Marlowe2003). In harsh (high latitude, cold climate) environments with low pathogen levels and low population density, polygynously and monogamously married women especially rely on male provision for raising their young. The kind of polygyny found in temperate climates with a high pathogen load is mainly gene-based rather than resource-based, and it involves little paternal provisioning or parenting (Low Reference Low1990). Because bi-parenting is essential for the evolution of human family sociality, it is seemingly difficult to argue for stronger family ties associated with gene-based polygyny with little paternal involvement compared to monogamy or resource-based polygyny with paternal provision.
However, a more detailed analysis would show an association between pathogen load and post-marital residence mode. High pathogen regions historically tend to have practiced matrilocality more than low pathogen regions. According to the Ethnographic Atlas (Murdock Reference Murdock1967), the matrilocal-to-patrilocal ratio (MPR) is .17 in Circum-Mediterranean (an area which includes today's Europe but also northern Africa). Today's Europe has a low historical pathogen load (HPL=–0.62) and a low current pathogen load (CPL=−2.88). The matrilocal-to-patrilocal ratio is much higher for high pathogen areas such as South and Central America (MPR=1.37; HPL=0.26; CPL=1.42), Insular Pacific (MPR=0.57; HPL=−0.24; CPL=−0.65), and sub-Saharan Africa (MPR=0.20; HPL=2.08; CPL=3.79). Similarly, matrilocal residence accounts for half of the historical societies in the Pacific islands (Jordan et al. Reference Jordan, Gray, Greenhill and Mace2009). Societies based on matrilocal rather than patrilocal residence should develop stronger and larger kinship networks due to reduced paternity uncertainty. Thus, pathogen stress may have a direct effect on post-marital residence, which in turn affects family ties and kinship relations. Close family ties and kinship interactions especially among maternal relatives promote a culture that favors reproduction and kinship association over non-reproductive and individualistic behaviors and attitudes constituting modernity (Newson & Richerson Reference Newson and Richerson2009; Newson et al. Reference Newson, Postmes, Lea and Webley2005). Modernity versus traditionalism also accounts for much of the variance in in-group sociality.
Second, relevant to the religiosity hypothesis, it should be noted that the vast majority of the world's population (55% of the global population) practice Christianity and Islam (The World Factbook, Central Intelligence Agency 2010). Few societies practice polytheistic beliefs today. Most of the data reported in the target article concern these two religions. Some of the most pathogen stressed regions include Africa (HPL=2.08; CPL=3.79), South and Central America (HPL=0.26; CPL=1.42), and a part of Asia represented by Indonesia (HPL=0.61; CPL=3.60) and the Philippines (HPL=0.50; CPL=1.64), all of whose populations mainly practice one of these two religions. These two religions originated from low pathogen areas, that is, today's Middle East countries (average HPL=0.14; average CPL=−0.90), and were brought to high-pathogen areas, not the other way around. The fact that populations under high rather than low pathogenic stress could be converted from their local beliefs and persuaded to embrace foreign religions seemingly contradicts the argument for a pathogen-driven in-group religious sociality including ethnocentrism and xenophobia. One explanation could be that these religions were brought to the peoples of these regions together with medicine and technology which helped to reduce the local normative pathogenic level at the time. Colonial brutality and failed local resistance could be another explanation.
We offer an alternative explanation that is framed within the cultural evolution theory (Boyd & Richerson Reference Boyd and Richerson2005). There are two broad adaptive strategies: social learning, or copying existing solutions; and individual learning, or innovating new ones (Boyd & Richerson Reference Boyd and Richerson2005). Adaptation toward one of these two strategic directions is normally conditioned by the extent to which new and old situations are similar enough to befit existing solutions. When the local environment is relatively stable, social learning prevails; whereas adaptive strategies tend to favor individual learning when a rapidly changing environment offsets the cost of trial and error. Pathogens affect environmental variability and human adaptive response in two ways. First, they add to environmental stability. The pathogenic level of a humanly habitable environment must not exceed the human physical immune threshold. A higher versus a lower pathogen load suggests that the mean pathogenic level is chronically closer to or father away from the human immune threshold, which sets the upper limit on the pathogenic distribution to result in a smaller (with a higher pathogen mean closer to threshold) versus larger (with a lower mean farther away from threshold) variability of the distribution. In accordance with this logic, a high pathogen load is associated with low environmental variability, which should elicit copying and social learning as a behavioral response. Second, the life threatening situation of infectious diseases raises the cost-benefit ratio of trial and error or individual learning. Copying or social learning, including its psychological facilitators, conformity and compliance, has been found to correlate with high pathogen stress both at the societal (Chang et al. Reference Chang, Mak, Li, Wu, Chen and Lu2011; Murray et al. Reference Murray, Trudeau and Schaller2011) and the individual level (Wu & Chang, under review). Copying existing solutions and conforming to local norms prove to be more adaptive than trial and error when dealing with infectious diseases (Murray et al. Reference Murray, Trudeau and Schaller2011). This pathogen specific adaptation spreads to other domains of life to result in high copying and high social conformity and compliance among people living under high pathogen conditions (Chang et al. Reference Chang, Mak, Li, Wu, Chen and Lu2011; Murray et al. Reference Murray, Trudeau and Schaller2011). Thus, so far in human history, new religions, technologies, and fashions have tended to be developed in low pathogen regions and brought to and copied by high pathogen regions, not the other way around.
Fincher & Thornhill (F&T) have presented cross-national and American data to support the hypothesis that pathogen stress is positively associated with family ties and religious affiliation constituting in-group sociality. Their evolutionary argument that pathogen stress is an ultimate cause of family and religious sociality is well grounded both in theory and in data. However, their broad-stroke analyses and explanations leave out two seemingly counterintuitive conclusions which we try to address by attending to more detailed analyses within the same pathogen-stress framework.
First, with respect to the hypothesis about pathogen-driven family ties, existing research based on the Standard Cross Culture Sample (Murdock & White Reference Murdock and White1969) shows a positive association between pathogen stress and polygyny (Katz & Konner Reference Katz, Konner and Lamb1981; Low Reference Low, Betzig, Borgerhoff Mulder and Turke1988; Reference Low1990; Marlowe Reference Marlowe2000; Reference Marlowe2003). In harsh (high latitude, cold climate) environments with low pathogen levels and low population density, polygynously and monogamously married women especially rely on male provision for raising their young. The kind of polygyny found in temperate climates with a high pathogen load is mainly gene-based rather than resource-based, and it involves little paternal provisioning or parenting (Low Reference Low1990). Because bi-parenting is essential for the evolution of human family sociality, it is seemingly difficult to argue for stronger family ties associated with gene-based polygyny with little paternal involvement compared to monogamy or resource-based polygyny with paternal provision.
However, a more detailed analysis would show an association between pathogen load and post-marital residence mode. High pathogen regions historically tend to have practiced matrilocality more than low pathogen regions. According to the Ethnographic Atlas (Murdock Reference Murdock1967), the matrilocal-to-patrilocal ratio (MPR) is .17 in Circum-Mediterranean (an area which includes today's Europe but also northern Africa). Today's Europe has a low historical pathogen load (HPL=–0.62) and a low current pathogen load (CPL=−2.88). The matrilocal-to-patrilocal ratio is much higher for high pathogen areas such as South and Central America (MPR=1.37; HPL=0.26; CPL=1.42), Insular Pacific (MPR=0.57; HPL=−0.24; CPL=−0.65), and sub-Saharan Africa (MPR=0.20; HPL=2.08; CPL=3.79). Similarly, matrilocal residence accounts for half of the historical societies in the Pacific islands (Jordan et al. Reference Jordan, Gray, Greenhill and Mace2009). Societies based on matrilocal rather than patrilocal residence should develop stronger and larger kinship networks due to reduced paternity uncertainty. Thus, pathogen stress may have a direct effect on post-marital residence, which in turn affects family ties and kinship relations. Close family ties and kinship interactions especially among maternal relatives promote a culture that favors reproduction and kinship association over non-reproductive and individualistic behaviors and attitudes constituting modernity (Newson & Richerson Reference Newson and Richerson2009; Newson et al. Reference Newson, Postmes, Lea and Webley2005). Modernity versus traditionalism also accounts for much of the variance in in-group sociality.
Second, relevant to the religiosity hypothesis, it should be noted that the vast majority of the world's population (55% of the global population) practice Christianity and Islam (The World Factbook, Central Intelligence Agency 2010). Few societies practice polytheistic beliefs today. Most of the data reported in the target article concern these two religions. Some of the most pathogen stressed regions include Africa (HPL=2.08; CPL=3.79), South and Central America (HPL=0.26; CPL=1.42), and a part of Asia represented by Indonesia (HPL=0.61; CPL=3.60) and the Philippines (HPL=0.50; CPL=1.64), all of whose populations mainly practice one of these two religions. These two religions originated from low pathogen areas, that is, today's Middle East countries (average HPL=0.14; average CPL=−0.90), and were brought to high-pathogen areas, not the other way around. The fact that populations under high rather than low pathogenic stress could be converted from their local beliefs and persuaded to embrace foreign religions seemingly contradicts the argument for a pathogen-driven in-group religious sociality including ethnocentrism and xenophobia. One explanation could be that these religions were brought to the peoples of these regions together with medicine and technology which helped to reduce the local normative pathogenic level at the time. Colonial brutality and failed local resistance could be another explanation.
We offer an alternative explanation that is framed within the cultural evolution theory (Boyd & Richerson Reference Boyd and Richerson2005). There are two broad adaptive strategies: social learning, or copying existing solutions; and individual learning, or innovating new ones (Boyd & Richerson Reference Boyd and Richerson2005). Adaptation toward one of these two strategic directions is normally conditioned by the extent to which new and old situations are similar enough to befit existing solutions. When the local environment is relatively stable, social learning prevails; whereas adaptive strategies tend to favor individual learning when a rapidly changing environment offsets the cost of trial and error. Pathogens affect environmental variability and human adaptive response in two ways. First, they add to environmental stability. The pathogenic level of a humanly habitable environment must not exceed the human physical immune threshold. A higher versus a lower pathogen load suggests that the mean pathogenic level is chronically closer to or father away from the human immune threshold, which sets the upper limit on the pathogenic distribution to result in a smaller (with a higher pathogen mean closer to threshold) versus larger (with a lower mean farther away from threshold) variability of the distribution. In accordance with this logic, a high pathogen load is associated with low environmental variability, which should elicit copying and social learning as a behavioral response. Second, the life threatening situation of infectious diseases raises the cost-benefit ratio of trial and error or individual learning. Copying or social learning, including its psychological facilitators, conformity and compliance, has been found to correlate with high pathogen stress both at the societal (Chang et al. Reference Chang, Mak, Li, Wu, Chen and Lu2011; Murray et al. Reference Murray, Trudeau and Schaller2011) and the individual level (Wu & Chang, under review). Copying existing solutions and conforming to local norms prove to be more adaptive than trial and error when dealing with infectious diseases (Murray et al. Reference Murray, Trudeau and Schaller2011). This pathogen specific adaptation spreads to other domains of life to result in high copying and high social conformity and compliance among people living under high pathogen conditions (Chang et al. Reference Chang, Mak, Li, Wu, Chen and Lu2011; Murray et al. Reference Murray, Trudeau and Schaller2011). Thus, so far in human history, new religions, technologies, and fashions have tended to be developed in low pathogen regions and brought to and copied by high pathogen regions, not the other way around.