No sex difference in digit ratios (2D:4D) in the traditional
Yali of Papua and its meaning for the previous hypotheses
on the inter-populational variability in 2D:4D
University of Wroclaw, Institute of
Psychology, ul. Dawida, Wroclaw,
Michalina Marczak, Institute of
Psychology, ul. Dawida 1, 50-527,
Polish National Science Centre
(SONATA BIS grant 2014/14/E/HS6/
00368 to Piotr Sorokowski)
Objectives: The ratio between the 2nd and 4th digit (2D:4D) is considered to be a
sexually dimorphic trait in humans, with males having on average lower 2D:4D val-
ues than females. However, significant inter- and intra-sexual differences in digit
ratios across populations have been reported. In order to further explore factors influ-
encing the variability of 2D:4D, we performed a study among the Yali, a traditional
population from Papua.
Methods: We measured digit ratios of 79 adults (47 males) from the traditional Yali
society, a polygamous group who inhabit a harsh high-mountain environment almost
directly on the equator.
Results: Statistical analysis of the data show no significant inter-sexual difference in
digit ratios in our sample.
Conclusions: We discuss the results in light of various factors that putatively shape
the differences in digit ratios. We conclude that the results of our study contribute to
the existing evidence suggesting that digit ratio might not be universally sexually
dimorphic in humans.
The ratio of index (2nd) to ring (4th) finger (2D:4D, digit
ratio) putatively reflects the degree of the influence of sex
hormones in utero and is considered sexually dimorphic in
humans (Manning, Scutt, Wilson, & Lewis-Jones, 1998).
Adult males have on average lower 2D:4D values than
female adults (e.g., Manning et al., 1998). Out of hundreds
of studies on digit ratio, only a few indicated lack of sexual
dimorphism regarding this trait (e.g., Austrians from Vora-
cek, Pietschnig, Nader, & Stieger, 2011; Zulu from Manning,
Henzi, Venkatramana, Martin, & Singh, 2003; Dutch sample
from Rammsayer & Troche, 2007; and the Hadza hunter-
gatherers from Apicella, Tobolsky, Marlowe, & Miller,
2016, but see Butovskaya, Burkova, Karelin, & Fink, 2015
for opposite results in the Hadza).
Evidence from various populations shows strong influ-
ence of ethnicity on inter- and intra-sexual differences in
2D:4D (Manning et al., 2000). Manning proposed that this
ethnic variance in 2D:4D might be explained by a curvilinear
function of latitude, with people living at intermediate lati-
tudes having in general higher digit ratios and showing
greater sex difference in 2D:4D than their counterparts at the
equator and high latitudes (2002). The evidence supporting
this hypothesis, however, is inconclusive (compare: Helle &
Laaksonen, 2009; Hurd & van Anders, 2007).
In harsh environments, both sexes seem to prefer more
physiologically masculine partners (DeBruine, Jones, Craw-
ford, Welling, & Little, 2010). In difficult conditions, where
competition for resources is high, females must undertake
competitive efforts along with men in order to survive hence
their greater masculinity (Cashdan, 2008). Furthermore, in
Am J Hum Biol. 2018;30:e23078.
2017 Wiley Periodicals, Inc.
Received: 2 April 2017
Revised: 19 October 2017
Accepted: 24 October 2017
American Journal of Human Biology