This document summarizes a study conducted by Dr. Geoff Kushnick on marriage practices among the Karo Batak people in North Sumatra, Indonesia. The study tested three hypotheses related to the Westermarck hypothesis and how levels of disgust toward marrying cousins (called "impal" marriage) may have changed over time. Results supported all three hypotheses, finding that individuals who were cosocialized with cousins felt more disgust toward impal marriage, descriptions of cosocialized cousins elicited more disgust, and male respondents showed increasing disgust toward impal marriage over time compared to female respondents. Further analysis is needed to understand why male attitudes have changed but not female attitudes.

1. Disgust and Choosiness:
A Biocultural Study of Social Change
Dr Geoff Kushnick  School of Archaeology and Anthropology  ANU College of Arts and Social Sciences

2. Impal Marriage Project Acknowledgments
Fulbright Scholars Program
American Institute for
Indonesian Studies
Karo people who participated
in the study
Dan Fessler
Anthropology, UCLA
Fikarwin Zuska
Anthropology
University of North Sumatra
Karmila Karo
Lasma Sinaga
Ikhsan Ginting

3. Schematic Diagram of
Matrilateral Cross-Cousin
(impal) Marriage among
the Karo Batak of North
Sumatra, Indonesia
Fessler DMT, 2007. Neglected
natural experiments germane to
the Westermarck hypothesis: the
Karo Batak and the Oneida
community. Hum Nat, 18, 355-64.
Kushnick G, Fessler DMT, 2011.
Karo Batak cousin marriage,
cosocialization, and the
Westermarck hypothesis. Current
Anthropology, 52(3), 443-448.

4. % of Total Marriages between Impal
Kushnick G, Fessler DMT, 2010. Study of marriage and baptism records at Catholic
Parish, Kabanjahe, North Sumatra. Center for Culture, Brain, and Development, UCLA.

5. • Edvard Westermarck
• Finnish sociologist
• 1862-1939
• “The History of Human
Marriage” (1891)
“Natural Experiments”
Showing Negative Imprinting
(aka Westermarck Effects)
Israeli kibbutzim
Taiwanese sim pua marriage

6. • Disgust evolved as a
mechanism to avoid
disease-causing agents
• Co-opted for two other
functions related to:
• Mating
• Morality
Tybur JM, Lieberman D, Griskevicius V, 2009. Microbes, mating, and morality: Individual
differences in three functional domains of disgust. J Pers Soc Psychol, 97, 103-122.
Tybur JM, Lieberman D, Kurzban R, DeScioli P, 2013. Disgust: evolved function and
structure. Psychol Rev, 120, 65-84

7. Fessler DMT, Navarette CD,
2004. Third-party attitudes
toward sibling incest:
Evidence for Westermarck's
hypotheses. Evol Hum Behav,
25, 277-294.
Antfolk J, Karlsson M,
Bäckström A, Santtila P, 2012.
Disgust elicited by third-party
incest: the roles of biological
relatedness, co-residence, and
family relationship. Evol Hum
Behav, 33, 217-223.
Fessler & Navarette (2004):
Respondents cosocialized with cousins
more disgusted by vignettes featuring
incest.
Antfolk et al. (2012):
Respondents cosocialized with cousins
more disgusted by vignettes featuring
incest; also more disgusted when
incestuous couple in vignette cosocialized.

8. Vignette experiment:
• 215 Karo men and women age 18 to 95
from 51 villages
• Each respondent interviewed briefly
about coresidence history with impal
• Then, each presented with 4 vignettes
featuring hypothetical Karo women and
their impal
• In halfdescribed as having grown up in
adjacent households since they were
small
• In the other halfdescribed as growing
up in a very far away village
• Asked how disgusting it would be if the
woman and her impal married.

9. Hypothesis 1
Participants cosocialized with their impal
should experience more disgust. For this
hypothesis, cosocialization was operationalized as
number of years from birth to age 10 that the
respondent lived in the same village as at least one of his
or her impal.
Hypothesis 2
Participants should experience more disgust when presented with
vignettes that feature a woman marrying an impal with whom she
was cosocialized. For this hypothesis, cosocializaton was operationalized as growing
up in adjacent houses.
Hypothesis 3
There should be concomitant secular trend toward increasing
disgust for impal marriage in male and female respondents.

10. Proportion of Responses
Responses were
Poisson Distributed

12. H3 There should be concomitant secular trends toward increasing
disgust for impal marriage in male and female respondents.

14. Further study of already-collected data
will investigate why male feelings toward
impal marriage have changed but female
feelings have remained constant.
Why are males more choosy?
- Patriarchal values?
- Male over-valuation of attractiveness of
potential partners?