Results and Discussion

Our empirical identification strategy is based on a comparison of two distinct tribes in Northeast India (the Khasi and the Karbi) that share a genetic background. This comparison allows us to identify the role of nurture. In this region, geographic contiguity is a better predictor of genetic similarity than culture. Both tribes are located in the hills surrounding the city of Shillong, and the Karbi and Khasi appear to be close kin, based on genetic analysis of six polymorphic loci (17). The villagers in both societies are agriculturalist and subsist primarily off rice, with little variation in wealth or diet.

There are many cultural differences between the tribes; thus, we cannot isolate the particular aspect of nurture that matters. However, the most obvious difference between the tribes is that the Karbi are a patrilineal tribe (for example, women are not supposed to own land, and the oldest son inherits the property), whereas the Khasi are a matrilineal tribe (property is inherited by the youngest daughter, men are not allowed to own land, and any earnings of the male are supposed to be handed over to his wife or sister). The different societies are described in greater details elsewhere (18, 19).

The participants in the field experiment solved a four-piece puzzle (Fig. 1) (a total of 1,279 participants from four Khasi and four Karbi villages). None of them had previous experience with such a task. To incentivize participants, we offered 20 rupees—25% of a day's wage—to each participant who solved the puzzle in less than 30 s. Some of our subjects also answered survey questions.

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Fig. 1.

The puzzle used in the experiment. Subjects solved a four-piece jigsaw puzzle. The time that it took to complete the puzzle acted as our measure of spatial ability.

The null hypothesis that we test is that, although culture may have a main effect on time to solve the puzzle, it will not affect the gender gap. The alternative hypothesis is that the gender gap is less pronounced among the matrilineal society.

First, we confirm that the two societies treat men and women differently; our survey data show that women get a better education and are more likely to own property in the matrilineal society. In the matrilineal society, males and females have the same years of education (tobit, two-sided P = 0.159, n = 530), whereas in the patrilineal society, males have 3.67 y more years of education (tobit, P < 0.001, n = 426). Men predominantly own the property in which our patrilineal participants dwelled, with only 35 of 347 exceptions. However, male ownership is strictly forbidden among the matrilineal villages. Details on these regressions and all others are in SI Text.

Our main result, the average time of each group to solve the puzzle, is presented in Fig. 2 and Table 1. As can be seen from Fig. 2 and Table 1 (consistent with other cross-cultural studies) (15), we find a main effect of culture on spatial abilities; however, unlike other cross-cultural studies, Fig. 2 and Table 1 also indicate an interaction effect between gender and culture. Men take 36.4% less time than women among the patrilineal society (ordinary least squares, P < 0.001, n = 468) but are no faster among the matrilineal society (OLS, P = 0.252, n = 811). The interaction between gender and society is statistically significant (OLS, P < 0.001, n = 1,279). SI Text shows the robustness of these results.

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Fig. 2.

Female villagers took longer to solve the puzzle in the patrilineal society but no longer in the matrilineal society. Mean time to solve the puzzle by society and gender. Subjects solved a four-piece jigsaw puzzle. The time that it took to complete the puzzle acted as our measure of spatial ability.

Education could be one mechanisms involved in generating the interaction between society, gender, and spatial abilities. This question has policy relevance, because policymakers have sway over education. Recall that there is a gender gap in education in the patrilineal but not the matrilineal society. Better educated subjects solve the puzzle faster, with each 1 y corresponding to a reduction in time of 4.3% (OLS, P < 0.001, n = 956). This finding is consistent with findings elsewhere that spatial abilities correlate with education (16). We find that, when adding education as a control variable, the gender by society interaction term remains significant (OLS, P < 0.001, n = 956) and quite sizeable, amounting to the effect of 6 y education, but it is reduced in magnitude by nearly a one-third, indicating that our main result is partly but not fully explained by education.

Next, we test whether household variation within a village matters. This question has practical implications, because it may be easier for ordinary people to influence household variation than alter an entire society. In particular, we test whether the gender gap in time to solve the puzzle differs between homes owned by males, which is traditional, and homes owned by females or jointly owned by males and females. The gender gap is, in fact, one-third the size among those people who live in homes not owned solely by males (10% OLS, P = 0.735, n = 35 vs. 42% OLS, P < 0.001, n = 312). Of course, this finding might be the result of selection into such household. A similar comparison cannot be made among matrilineal households, because men are outright forbidden to own property, preventing variation.

Recall that the youngest daughter traditionally inherits property among the Khasi and the oldest son traditionally inherits property among the Karbi. One might worry that our results are being driven solely by the privileged individuals. However, the youngest daughters were not significantly faster among Khasi females (OLS, P = 0.80, n = 176) and the oldest sons were not significantly faster among Karbi males (OLS, P = 0.47, n = 203). Moreover, if we restrict our analysis to subjects who are not these privileged inheritors, our main result remains unchanged.

It has earlier been shown (18–20) that, although men are generally more competitive than women, this is not the case in the matrilineal society. Thus, one may worry that time to solve the puzzle is merely picking up on competitiveness, and the current interaction between gender and society in time to solve the puzzle is caused by the interaction between gender and society in competitiveness. This conclusion is not the case. Using the same measure as in an earlier work (described in SI Text), we find no relationship between competitiveness and time to solve the puzzle (OLS, P = 0.864, n = 976), and including competitiveness as a control does not influence our main result (18).