www.epjournal.net – 2007. 5(1): 47-51
Distinguishing Byproducts from Non-Adaptive Effects of Algorithmic
Justin H. Park, Department of Psychology, University of Groningen, Groningen, The Netherlands. Email:
Abstract: I evaluate the use of the byproduct concept in psychology, particularly the
adaptation–byproduct distinction that is commonly invoked in discussions of psychological
phenomena. This distinction can be problematic when investigating algorithmic
mechanisms and their effects, because although all byproducts may be functionless
concomitants of adaptations, not all incidental effects of algorithmic adaptations are
byproducts (although they have sometimes been labeled as such). I call attention to
Sperber’s (1994) distinction between proper domains and actual domains of algorithmic
mechanisms. Extending Sperber’s distinction, I propose the terms adaptive effects and
non-adaptive effects, which more accurately capture the phenomena of interest to
psychologists and prevent fruitless adaptation-versus-byproduct debates.
Keywords: algorithmic adaptation, byproduct, actual domain, proper domain, adaptive
effect, non-adaptive effect.
The adaptation concept has been immensely useful in psychology. Theories and
hypotheses derived from adaptationist thinking have produced an abundance of discoveries
that have filled several recent edited volumes (e.g., Buss, 2005; Dunbar and Barrett, 2007;
Schaller, Simpson, and Kenrick, 2006). The byproduct concept, despite being the focus of
substantial discussion and exposition (e.g., Andrews, Gangestad, and Matthews, 2002;
Buss, Haselton, Shackelford, Bleske, and Wakefield, 1998; Klein, Cosmides, Tooby, and
Chance, 2002), has been far less useful, at least with respect to empirical yield. Moreover,
as I show below, the concept has sometimes been applied in a manner inconsistent with its
definition. In this article, I evaluate the use of the byproduct concept in psychology—
particularly the associated idea that commonly investigated psychological phenomena can
be categorized as either adaptations or byproducts. I argue that the adaptation–byproduct
distinction often fails to capture the distinction of interest to psychologists, and I advocate
the use of another distinction based on Sperber (1994).
Byproducts and non-adaptive effects
What is wrong with the adaptation–byproduct distinction? First, what is a
byproduct? According to established definition, a byproduct is a trait that evolved “not
because it was selectively advantageous, but because it was inextricably linked (either
through pleiotropy or linkage disequilibrium) to another trait that was reproductively
advantageous” (Andrews et al., 2002, p. 491; for a similar definition, see Buss et al., 1998).
A commonly invoked analogy for byproducts is architectural spandrels, which are spaces
that are necessarily left between designed features of buildings (Buss et al., 1998; Gould,
1997). There are several examples of biological byproducts that clearly conform to this
definition, such as the whiteness of bone and the belly button—these features evolved, are
functionless, and are inextricably linked to an adaptation (i.e., white bones are necessary
effects of selecting for bone made of calcium).
A number of evolutionary psychologists have applied the adaptation–byproduct
distinction by asking questions of the form “Is X an adaptation, or is it a byproduct of an
adaptation?” where X represents phenomena such as automatic encoding of race or rape
(e.g., Kurzban, Tooby, and Cosmides, 2001; Thornhill and Palmer, 2000). According to
Kurzban et al., humans possess adaptations for tracking coalitional alliances, which are
sensitive to a variety of information suggesting group membership. In modern contexts
consisting of racially diverse individuals, race-related physical features may (incorrectly)
be perceived as cues for coalitional alliances, resulting in the automatic encoding of race.
Kurzban et al. went on to show that automatic encoding of race can be eliminated by
modifying contextual circumstances. Therefore, they concluded, the tendency to
automatically encode race is a byproduct, not an adaptation.
That conclusion, however, depends on a looser, more colloquial use of the term
‘byproduct.’ Although encoding race may indeed be an incidental effect of an adaptation
for tracking coalitional alliances, it is not an evolved feature and is demonstrably not
inextricably linked to this adaptation—it is just one possible effect in one possible social
milieu. The confusion arises because (1) the term byproduct has been used in a different
manner and (2) effects of functional algorithms have been conflated with the algorithmic
mechanisms themselves. Unlike structural traits—such as bones and umbilical cords—
behavioral ‘traits’ that many psychologists tend to investigate are information-processing
algorithms that take in specific stimuli as input and produce specific psychological or
behavioral responses as output.
As shown in Figure 1, a psychological adaptation designed to detect coalitional
alliances consists of a functional algorithmic component that detects cues that potentially
signal coalitional alliances and translates those cues into quick categorization of the social
world. In different social contexts, this algorithmic mechanism may detect a range of
coalition-connoting cues (some diagnostic, others not) and produce a response (some
functional, others not). The functional component of this adaptation is the algorithmic
mechanism (depicted in the box in Figure 1). Thus, one can claim to have identified a
byproduct of this adaptation only when one has identified some evolved feature that is
inextricably linked to this algorithmic mechanism. The various effects of this algorithmic
adaptation—whether they are adaptive effects or incidental effects—are not properly the
focus of the adaptation-versus-byproduct debate, because they logically cannot be either.
Therefore, asking whether encoding race is an adaptation or a byproduct is misleading.
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(1). 2007. -48-
Byproducts and non-adaptive effects
Figure 1. A schematic diagram of coalitional-alliance detection adaptation, which detects a
range of stimuli (some diagnostic, others not) and translates them into specific responses
(some functional, others not), a pattern that is characteristic of many algorithmic
As another example, Thornhill and Palmer (2000) asked whether rape is an
adaptation or a byproduct of other adaptations—specifically, men’s desire for sex and
tendency to use violence in pursuit of a goal. The authors were undecided, but for the
moment, suppose that rape is not an adaptation, but is an incidental effect of these other
adaptations; that is, given some environmental conditions, these adaptations sometimes
produce effects that lead to rape. Then, rape would represent just one possible effect of
these adaptations in a given circumstance. And as it would not represent an evolved feature
that is inextricably linked to those adaptations, it should not be labeled a byproduct.
Indeed, given the multiplicity of psychological mechanisms that may underlie and
contribute to rape, the adaptation-versus-byproduct dichotomy may have been too
simplistic to begin with (Pinker, 2002).
If in fact phenomena such as automatic encoding of race and rape are neither
adaptations nor byproducts, what are they? Sperber (1994) offered a distinction that is less
known than the adaptation–byproduct distinction, but is perhaps more useful for
psychologists. He noted that algorithmic mechanisms (those that detect specific stimuli
and produce specific responses) can be said to have proper domains and actual domains.
The coalition-alliance detecting mechanism evolved in response to various indicators of
coalitional alliances—for example, “patterns of coordinated action, cooperation, and
competition” (Kurzban et al., 2001, p. 15387); these cues constitute the proper domain of
this adaptive mechanism. However, once in place, this mechanism may potentially respond
to a wider range of cues, such as team sports jerseys and race-related physical features,
regardless of whether these cues existed ancestrally or are diagnostic of coalitional
alliances; these cues constitute the actual domain of this adaptive mechanism.
Extending Sperber’s (1994) distinction, I propose a distinction between adaptive
effects (e.g., responding to ancestrally existing “proper” cues) and non-adaptive effects
(e.g., responding to evolutionarily novel “actual” cues). Then, automatic encoding of race
can be characterized as a non-adaptive effect of the coalition-alliance detecting adaptation.
The distinction isn’t perfect, because one could argue that some evolutionarily novel, ‘non-
proper’ cues—such as sports team jerseys—are actually diagnostic of coalitional alliances
and produce what appear to be ‘adaptive’ effects. Nevertheless, this distinction prevents
misuse of the byproduct concept and forces one to be more explicit about adaptive
components of algorithmic mechanisms and their effects. This is beneficial, because a
clear separation of algorithmic adaptations from their effects can prevent awkward
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(1). 2007. -49-
Byproducts and non-adaptive effects
conclusions such as that a coalition-alliance detecting adaptation, in the case of encoding
race has transformed into a byproduct (of course, the underlying adaptation has remained
unchanged and no real byproduct has been identified).
An important point for psychologists (especially those interested in algorithmic
stimulus–response mechanisms) is this: Not all psychological phenomena qualify for the
adaptation-versus-byproduct debate. The adaptation–byproduct distinction does not neatly
“carve nature at its joints” when investigating the nature and consequences of adaptive
algorithmic mechanisms. Several psychological phenomena—the tendency to overeat, the
tendency to stigmatize unusual-looking people, the tendency to react prosocially toward
facially similar (but unrelated) others, and the tendency to find unrelated housemates
sexually aversive—can be understood as incidental effects of algorithmic adaptations (e.g.,
DeBruine, 2005; Park, Faulkner, and Schaller, 2003; Pinel, Assanand, and Lehman, 2000;
Shepher, 1971). But asking whether these phenomena are adaptations or byproducts would
have missed the point, as they are neither; they are all effects of normally operating
In sum, when applying evolutionary reasoning to algorithmic psychological
processes, one must first separate the adaptive components (e.g., coalition-alliance
detecting mechanisms) from their effects (e.g., automatic encoding of race). Then, one
should attempt to identify the design features of the mechanism (including the
identification of the proper domain of the mechanism), which will then lead to hypotheses
about how the mechanism may operate in novel circumstances, producing various adaptive
and non-adaptive effects.
Does this mean that the byproduct concept is useless in the study of algorithmic
adaptations? That remains an empirical issue. It’s possible that there are true byproducts
of algorithmic adaptations yet to be identified. In the meantime, we should be mindful of
our terminology so that we can recognize a real byproduct when we see one.
Acknowledgements: I thank an anonymous reviewer for advising me to discuss Sperber’s
Received 26 September 2006; revision submitted 13 December 2006; accepted 13
Andrews, P. W., Gangestad, S. W., and Matthews, D. (2002). Adaptationism – how to
carry out an exaptationist program. Behavioral and Brain Sciences, 25, 489-553.
Buss, D. M. (Ed.). (2005). The Handbook of Evolutionary Psychology. Hoboken, NJ:
Buss, D. M., Haselton, M. G., Shackelford, T. K., Bleske, A. L., and Wakefield, J. C.
(1998). Adaptations, exaptations, and spandrels. American Psychologist, 53, 533-
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(1). 2007. -50-
Byproducts and non-adaptive effects
DeBruine, L. M. (2005). Trustworthy but not lust-worthy: Context-specific effects of facial
resemblance. Proceedings of the Royal Society B, 272, 919-922.
Dunbar, R. I. M., and Barrett, L. (Eds.). (2007). Oxford handbook of evolutionary
psychology. Oxford, UK: Oxford University Press.
Gould, S. J. (1997). The exaptive excellence of spandrels as a term and prototype.
Proceedings of the National Academy of Sciences, 94, 10750-10755.
Klein, S. B., Cosmides, L., Tooby, J., and Chance, S. (2002). Decisions and the evolution
and memory: Multiple systems, multiple functions. Psychological Review, 109,
Kurzban, R., Tooby, J., and Cosmides, L. (2001). Can race be erased? Coalitional
computation and social categorization. Proceedings of the National Academy of
Sciences, 98, 15387-15392.
Park, J. H., Faulkner, J., and Schaller, M. (2003). Evolved disease-avoidance processes and
contemporary anti-social behavior: Prejudicial attitudes and avoidance of people
with physical disabilities. Journal of Nonverbal Behavior, 27, 65–87.
Pinel, J. P. J., Assanand, S., and Lehman, D. R. (2000). Hunger, eating, and ill health.
American Psychologist, 55, 1105-1116.
Pinker, S. (2002). The Blank Slate: The Modern Denial of Human Nature. New York:
Schaller, M., Simpson, J. A., and Kenrick, D. T. (Eds.). (2006). Evolution and Social
Psychology. New York: Psychology Press.
Shepher, J. (1971). Mate selection among second generation kibbutz adolescents and
adults: Incest avoidance and negative imprinting. Archives of Sexual Behavior, 1,
Sperber, D. (1994). The modularity of thought and the epidemiology of representations. In
L. A. Hirschfeld and S. A. Gelman (Eds.), Mapping the Mind: Domain Specificity
in Cognition and Culture (pp. 39-67). Cambridge, UK: Cambridge University Press.
Thornhill, R., and Palmer, C. T. (2000). A Natural History of Rape: Biological Bases of
Sexual Coercion. Cambridge, MA: MIT Press.
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(1). 2007. -51-