Original Article
Rethinking the Taiwanese minor marriage data: evidence the mind uses multiple kinship cues to regulate inbreeding avoidance

https://doi.org/10.1016/j.evolhumbehav.2008.11.003Get rights and content

Abstract

Natural experiments such as the Israeli Kibbutzim and Taiwanese minor marriages provide unique opportunities for testing the effects of childhood association on adult sexual attraction. Within these populations, early childhood association leads to the development of a sexual aversion, an effect first proposed by Edward Westermarck. However, recent analysis of Taiwanese minor marriages indicates that only the age at first association (an inverse index of childhood association) of the younger partner predicts marital fertility rates; the age at first association of the older partner does not. Although considered a puzzle, a recent model of human inbreeding avoidance can explain this pattern. This model suggests that the mind uses at least two kinship cues to regulate the development of sibling sexual aversions: (i) childhood coresidence duration, a default cue used mainly by younger siblings in detecting probable older siblings, and (ii) exposure to one's mother caring for a newborn, a cue only available to older siblings and reliable regardless of coresidence duration and, hence, age at first association. Thus, one reason that the age at first association of only the younger partner in minor marriages predicts fertility is that coresidence duration serves as a cue to siblingship mainly for younger partners; older partners use a different kinship cue not influenced by durations of association. When compared to data from psychological investigations of the effects of coresidence duration on opposition to sibling incest, the minor marriage data reveal an identical pattern providing converging lines of evidence that multiple kinship cues mediate sibling detection and inbreeding avoidance in humans.

Introduction

Natural experiments such as those created by Taiwanese minor marriages and Israeli Kibbutzim provide valuable strands of data that shed light on the evolved structure of human inbreeding avoidance mechanisms. In particular, these unique arrangements allow researchers to investigate whether the hypothesis originally advanced by Edward Westermarck—that early childhood association leads to the development of a sexual aversion later during adulthood (Westermarck, 1891/1922)—applies even in situations where genetically unrelated individuals are raised together from infancy. Indeed, individuals who live in close physical association from early childhood tend not to marry one another as in Israeli Kibbutzim (Shepher, 1983) and, if forced to do so as in Taiwanese minor marriages, produce fewer offspring, engage in more extramarital affairs, and are more likely to get divorced compared to couples not raised together as children (Wolf, 1995). In general, these patterns support Westermarck's claim that childhood association serves as an ancestrally reliable cue to kinship and regulates the development of sexual aversions (see also Bevc & Silverman, 1993, Bevc & Silverman, 2000, Fessler & Navarrete, 2004, Lieberman et al., 2003, Lieberman et al., 2007).

However, follow-up data analyses from one natural experiment, Taiwanese minor marriages, indicate that durations of childhood association do not predict sexual aversions for both the older and younger partner in the marriage. Rather, Wolf, 1995, Wolf, 2005 finds that it is only the age at first association of the younger member of the arranged marriage that predicts fertility rates, a proxy used as an index of sexual aversions. Age at first association is an inverse index of coresidence duration and childhood association—the earlier the age an individual is introduced into the family, the longer the period of childhood coresidence; the later the age, the shorter the duration. Although Wolf (2005) attempts an explanation for this asymmetry, he confesses that this difficult question “is one of the feats the Hercules who tackles incest avoidance must perform” (p. 12).

As this article discusses, recent psychological investigations into the cognitive architecture of inbreeding avoidance mechanisms provide an explanation for why the age at first association—or coresidence duration—of the younger member, not the older member, of a pair predicts the intensity of sexual aversion (Lieberman et al., 2007). Re-evaluation of Wolf's Taiwanese minor marriage data in light of the model proposed by Lieberman et al. provides additional evidence that the mind uses multiple kinship cues to regulate the development of sexual aversions toward siblings.

Section snippets

Taiwanese minor marriage data

The research conducted by Wolf, 1966, Wolf, 1968, Wolf, 1970, Wolf, 1993, Wolf, 1995 and Wolf and Durham (2005) on Taiwanese minor marriage is perhaps the most extensive investigation of how sexual inhibitions develop to date. Taking advantage of the demographic records kept by the Japanese colonial government during their occupation of Taiwan between 1895 and 1945, Wolf was able to collect information on births, deaths, divorces, adoptions, and marriages from inhabitants on the island (Wolf,

A possible solution to Wolf's puzzle

The puzzle posed by the different patterns generated by older and younger partners in minor marriages can be solved by considering the structure of ancestral social environments and the kinds of kinship cues natural selection might have used to engineer an inbreeding avoidance mechanism. Recently, a model of inbreeding avoidance that takes this approach was advanced by Lieberman et al. (2007). According to this model, there exists a kin detection mechanism that takes as input cues that were

Conclusion

In two completely different cultures and in two completely different types of data sets (one archival and one survey), the same exact pattern is found in how coresidence duration and exposure to maternal–neonatal association mediate the development of sexual aversions between younger and older children raised in the same nuclear family. These data provide additional evidence that the mind uses (at least) two cues to identify siblings and to regulate the development of sibling sexual aversions.

Acknowledgments

I would like to thank H.C. Barrett, D. Fessler, R. Oum, A. Aylward and two anonymous reviewers for helping to significantly improve the final version of this manuscript.

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