ORIGINAL RESEARCH ARTICLE
Separating the Bruce and Trivers-Willard effects in theory
School of Public Health, University of
California, Berkeley, Berkeley, California
Department of Demography, University
of California, Berkeley, Berkeley,
Department of Biology, Santa Clara
University, Santa Clara, California 95053
Ralph Catalano, School of Public Health,
University of California, Berkeley,
Berkeley, California 94704.
This work was supported in part by the
Robert Wood Johnson Foundation
Objectives: Theories of reproductive suppression predict that natural selection would
conserve mechanisms that abort the gestation of offspring otherwise unlikely to thrive
in prevailing environments. Research reports evidence among humans of at least two
such mechanisms—the Trivers-Willard and Bruce Effects. No literature, however,
compares the mechanisms nor estimates their relative contribution to observed char-
acteristics of human birth cohorts. We describe similarities and differences between
the Trivers-Willard and Bruce Effects and explore high quality historical data from
Sweden to determine which mechanism better describes temporal variation in the
ratio of males to females in birth cohorts.
Methods: We measure Trivers-Willard exposures with the death rate among women
of reproductive age. We measure Bruce exposures with the death rate among chil-
dren. We use time-series regression methods to estimate the relative contribution of
the Trivers-Willard and Bruce Effects to temporal variation in historical Swedish sec-
ondary sex ratio data.
Results: We find that the Bruce Effect appears to be a better predictor of the second-
ary sex ratio than does the Trivers-Willard Effect.
Conclusions: Attempts to identify mechanisms by which reproductive suppression
affects fetal loss and characteristics of human birth cohorts should consider the Bruce
Effect as an alternative to the Trivers-Willard Effect.
Theories of reproductive suppression posit that natural
selection conserved mechanisms that reduce fertility when
offspring would otherwise fail to thrive (Beehner & Lu,
2013; Haig, 1999; Wasser & Barash, 1983). This paper
focuses on two mechanisms, the Bruce (1959) and Trivers
and Willard (1973) Effects, thought to implement reproduc-
tive suppression in several species, including humans.
Although the lines of literature describing these mecha-
nisms rarely acknowledge each other, they have much in
common. Both, for example, assume that pregnant women
vary in their capacity to invest in children and that children
vary in their need for maternal investment to thrive in
prevailing environments. Both mechanisms also assume
that low-resource mothers with high-need children will
have relatively few grandchildren because their children
will more frequently die before reproductive age than will
the children of other mothers. These shared assumptions
lead to the inference that natural selection would conserve
mutations that spontaneously abort gestations in which the
needs of the prospective child would otherwise exceed the
resources of the prospective mother.
The arguments embedded in these two lines of literature
also have important differences. We describe these differen-
ces and explore historic data to determine which argument
better describes temporal variation in an important character-
istic of human birth cohorts—the secondary sex ratio.
Am J Hum Biol. 2018;30:e23074.
2017 Wiley Periodicals, Inc.
Received: 28 April 2017
Revised: 21 August 2017
Accepted: 12 October 2017
American Journal of Human Biology