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The Evolutionary Psychology Lab is currently pursuing several programmatic
lines of research. This research has the overarching goal of gaining a clearer
and more comprehensive understanding of human sexual psychology and behavior.
A special focus of research in the Lab is on sexual conflict between men and
women.
The Evolutionary Psychology
Lab has been supported by research
contracts and grants provided by several funding sources, including the following:
Human sperm competition and sexual conflict
My colleagues and I have recently completed several studies testing evolutionary
psychological hypotheses about human sexual behavior and psychology and about
sexual conflict between men and women. Sperm competition occurs when the sperm
of two or more males simultaneously occupy the reproductive tract of a female
and compete to fertilize an egg. My colleagues and I (Shackelford et al., 2002, Evolution
and Human Behavior) used a questionnaire to investigate psychological responses
to the risk of sperm competition for 194 men in committed, sexual relationships
in the United States and in Germany. As predicted, a man who spends a
greater (relative to a man who spends a lesser) proportion of time apart from
his partner since the couple’s last copulation reported (a) that his partner is more
attractive, (b) that other men find his partner more attractive, (c) greater
interest in copulating with his partner, and (d) that his partner is more sexually
interested in him. All effects were independent of total time since the couple’s
last copulation and the man’s relationship satisfaction. This line of
research was funded by a grant to me from NIMH. We have recently published
or had accepted for publication several additional papers that investigate
empirically and theoretically human sexual psychology and behavior from the
perspective of sperm competition theory (e.g., Shackelford & Goetz, in
press, Journal of Comparative Psychology; Shackelford et al., 2005, Review
of General Psychology; Shackelford et al., 2004, Archives of Sexual
Behavior; Goetz, Shackelford, et al., 2005, Personality and Individual
Differences; Shackelford, 2003, Evolution and Cognition; Shackelford & LeBlanc,
2001, Evolution and Cognition).
In another line of research on human sexuality, but one specifically designed
to investigate an area of intense conflict between the sexes, I (Shackelford,
2001, Aggressive Behavior; and see also Shackelford, 2002, Journal
of Criminal Justice) investigated several key predictors of rape-murder.
Working from an evolutionary psychological perspective, M. Wilson, M.
Daly, and J. Scheib (1997) hypothesized and found that reproductive age
women incur excess risk of rape-murder (being raped and murdered), relative
to non-reproductive age girls and women, and that this excess risk cannot be
attributed solely to the greater association of young women with violent, young
men. This research provided the first national-level replication of these findings
for the United States. I secured access to a national database of homicides
occurring in the United States between 1976 and 1994 and selected for analysis
cases in which a girl or women was (a) raped and murdered by a man previously
unknown to her or (b) murdered in the context of theft by a man previously
unknown to her. The results replicated the work of Wilson et al. (1997) and
documented that (a) young men commit the majority of rape-murders and theft-murders;
(b) young, reproductive age women are over-represented among the victims of
rape-murder, but (c) are under-represented among the victims of theft-murder.
In the Discussion, I acknowledge the uncertain generalizability of theoretical
and empirical work on rape-murder to rape not accompanied by murder and I address
two challenges to an evolutionary perspective on rape-murder: (a) Why
are non-reproductive age girls and women raped?, and (b) Why are raped women
subsequently murdered? Although consistent with an evolutionary perspective
and contrary to a routine activities perspective, these results raise questions
that challenge a simple evolutionary interpretation. These questions cannot
be addressed by analyses of the current data, but demand attention by researchers
if we hope to reduce the risk of rape and rape-murder for all girls and women.
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Jealousy and infidelity
My colleagues and I have investigated several dimensions of the psychology
of jealousy and infidelity, guided by an evolutionary social psychological
perspective. The different adaptive problems faced by men and women over evolutionary
history led evolutionary psychologists to hypothesize and discover sex differences
in jealousy as a function of infidelity type. An alternative hypothesis proposes
that beliefs about the conditional probabilities of sexual and emotional infidelity
account for these sex differences. My colleagues and I (Buss, Shackelford,
et al., 1999, Personal Relationships) conducted four studies to test
these hypotheses (and see Shackelford, Buss, & Bennett, 2002, Cognition
and Emotion). Study 1 tested the hypotheses in a USA sample (N =
1,122) by rendering the types of infidelity mutually exclusive. Study 2 tested
the hypotheses in a USA sample (N = 234) by asking participants to identify
which aspect of infidelity was more upsetting when both forms occurred, and
by using regression to identify the unique contributions of sex and beliefs.
Study 3 replicated Study 1 in a Korean sample (N = 190). Study 4 replicated
Study 2 in a Japanese sample (N = 316). Across the studies, the evolutionary
hypothesis, but not the belief hypothesis, accounted for: sex differences in
jealousy when the types of infidelity are rendered mutually exclusive; sex
differences in which aspect of infidelity is more upsetting when both occur;
significant variance attributable to sex, after controlling for beliefs; sex-differentiated
patterns of beliefs; and the cross-cultural prevalence of all these sex differences.
In a study conducted in collaboration with two students at FAU (Shackelford,
LeBlanc, & Drass, 2000, Cognition and Emotion), I sought to identify
emotional reactions to a partner’s sexual infidelity and emotional infidelity.
In a preliminary study, 53 participants nominated emotional reactions to a
partner’s sexual and emotional infidelity. In a second study, 655 participants
rated each emotion for how likely it was to occur following sexual and
emotional infidelity. Principal components analysis revealed 15 emotion components,
including Hostile/Vengeful, Depressed, and Sexually Aroused. We conducted repeated
measures analyses of variance on the 15 components, with participant sex as
the between-subjects factor and infidelity type as the within-subjects factor.
A main effect for sex obtained for 9 components. For example, men scored higher
on Homicidal/Suicidal, whereas women scored higher on Undesirable/Insecure.
A main effect for infidelity type obtained for 12 components. For example,
participants endorsed Nauseated/Repulsed as more likely to follow sexual infidelity
and Undesirable/ Insecure as more likely to follow emotional infidelity.
In addition to the above papers, I have published the results of my research
on jealousy and infidelity in top-tier scholarly journals such as Personal
Relationships (Buss, Shackelford, Choe, & Buunk, 2000; Bleske & Shackelford,
2001; Schmitt,…Shackelford, et al., 2004), Journal of Personality
and Social Psychology (Buss & Shackelford, 1997; Schmitt,…Shackelford,
et al., 2003; 2004), Journal of Cross-Cultural Psychology (Schmitt,…Shackelford,
et al., 2004), Personality and Social Psychology Bulletin (Shackelford & Buss,
1996, 1997; Schmitt & Shackelford, 2003), Evolution and Cognition (Shackelford,
2003; Shackelford & LeBlanc, 2001), Evolution and Human Behavior (Shackelford
et al., 2002), European Journal of Social Psychology (Shackelford, Michalski, & Schmitt,
2004) and Human Nature (Shackelford et al., 2004).
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Intimate partner violence and homicide
My colleagues and I have conducted a substantial amount of research designed
to test social psychological hypotheses about intimate partner violence
and homicide. This research already has generated more than a dozen papers
published or in press in prestigious scholarly journals (Breitman, Shackelford, & Block,
2005, Violence and Victims; and 2003a and 2003b, Research Bulletin
of the Illinois Criminal Justice Information Authority; Mouzos & Shackelford,
2004, Aggressive Behavior; Peters, Shackelford, & Buss, 2002, Violence
and Victims; Shackelford, Buss, & Peters, 2000, Violence and Victims;
Shackelford, 2000, 2001, 2002; Aggressive Behavior; Shackelford, 2001, Homicide
Studies; Shackelford, 2002; Journal of Criminal Justice; Shackelford,
Buss, & Weekes-Shackelford, 2003, Basic and Applied Social Psychology;
Shackelford & Mouzos, 2005, Journal of Interpersonal Violence).
In one study, my colleagues and I (Shackelford, Buss, & Peters, 2000) investigated
the relationship between a woman’s age, her husband’s age, and
the risk that he will murder her. Younger women, relative to older women, incur
elevated risk of uxoricide—being murdered by their husbands. Some evolutionary
theorists attribute this pattern to men’s evolved sexual proprietariness,
which inclines them to use violence to control women, especially those
high in reproductive value. Other evolutionary theorists propose an evolved
homicide module for wife killing. An alternative to both explanations is that
young women experience elevated uxoricide risk as an incidental byproduct of
marriage to younger men, who commit the majority of acts of violence. We used
a sample of 13,670 uxoricides to test these alternative explanations. Findings
show that (1) reproductive age women incur an elevated risk of uxoricide relative
to older women; (2) younger men are over-represented among uxoricide
perpetrators; and (3) younger women, even when married to older men, still
incur excess risk of uxoricide.
In another study, I (Shackelford, 2000, Aggressive Behavior) investigated
a related question—whether men married to some women are at greater risk
of being murdered by their wife than are men married to other women.
When a woman kills her husband, it is almost always an unplanned action of
self-defense against a battering husband or a last-ditch attempt to survive
a batterer's tyranny. Younger, reproductive age women are battered and killed
by husbands at higher rates than are older, post-reproductive age women. Because
husband-killing occurs in the context of self-defense or as a last-ditch effort
to survive, reproductive age women should kill their husbands at higher rates
than do post-reproductive age women. I used a sample of 8,077 husband-killings
to test this hypothesis. Results support the hypothesis, and document that
(a) the highest rates of husband-killing are for the youngest women; (b) the
youngest husbands are at greatest risk of being killed by their wife; (c) women
married to older men kill their husband at higher rates than do women married
to same-age men and women married to younger men; and (d) reproductive age
women kill their husband at higher rates than do post-reproductive age women
across two groups: women married to younger men and women married to older
men.
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Mate preferences and mate selection
In a study designed to identify the cultural evolution of mate preferences,
my colleagues and I (Buss, Shackelford, et al., 2001, Journal of Marriage
and the Family) investigated the change and stability of mate preferences
of American college students over the past half-century. The qualities people
believe are important in selecting a marriage partner afford one domain for
assessing human values. We examined the cultural evolution of these values
over more than half a century. Building upon existing data on mate preferences
collected in 1939 (N = 628), 1956 (N = 120), 1967 (N =
566), and 1977 (N = 316), we collected data using the same instrument
in 1984/1985 (N = 1,496) and in 1996 (N = 607) at geographically
diverse locations. Several changes in values were documented across the 57-year
span. Both sexes increased the importance they attach to physical attractiveness
in a mate. Both sexes, but especially men, increased the importance they attach
to mates with good financial prospects. Domestic skills in a partner plummeted
in importance for men. Mutual attraction and love climbed in importance for
both sexes. The sexes converged in the ordering of the importance of different
mate qualities, showing maximum similarity in 1996.
Men universally express a preference for youth in a long-term mate, presumably
an evolved desire originating from the close and recurrent statistical
association between a woman’s age and her residual reproductive value (future reproductive
potential). Accordingly, my colleagues and I (Buss, Shackelford, & LeBlanc,
2000, Evolution and Human Behavior) hypothesized a positive correlation
for men (but not women) between the number of children desired and preferred
spousal age difference—a context-specific shift in mate preference depending
on whether the man is pursuing a “quality” or “quantity” reproductive
strategy. We tested this hypothesis with data provided by 9,808 participants
from 37 cultures located on six continents and five islands. Results
confirmed the hypothesis, even after statistically controlling for preferred
age at first marriage and current age of participant.
In addition to the above papers, I have published the results of my research
on mate preferences and mate selection in prestigious journals such as Evolution
and Human Behavior (Shackelford & Larsen, 1999; Shackelford et al.,
2002), Journal of Personality and Social Psychology (Shackelford & Larsen,
1997; Schmitt,…Shackelford, et al., 2003), Journal of Personality
(Botwin, Buss, & Shackelford, 1997), Human Nature (Shackelford,
et al., 2000), Personal Relationships (Schmitt, Shackelford, et al.,
2001; Schmitt,…Shackelford, et al., 2004), Evolution of Communication (Shackelford & Larsen,
2003), Canadian Journal of Human Sexuality (Schmitt, Shackelford, et
al., 2002), Journal of Research in Personality (Michalski & Shackelford,
2002), and Psychology, Evolution, and Gender (Schmitt, Shackelford, & Buss,
2001).
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Marital relationships
My colleagues and I have recently completed a long-term study of over 100
married couples. A general goal of this project was to better understand the
predictors of marital satisfaction, happiness, and longevity. Also of interest
to us were the ways in which characteristics of persons are predictive of marital
well being and, conversely, how characteristics of the marital relationship
are predictive of the well being of the spouses. In one study, I (Shackelford,
2001, Personality and Individual Differences) tested two hypotheses
about the function of self-esteem, with reference to the marital context: Self-esteem
evolved as a psychological solution to the adaptive problem of (1) tracking
reproductively-relevant costs inflicted by a spouse, and (2) tracking own value
as a long-term mate. Participants (N = 214) evaluated their self-esteem
and provided information about marital conflict and marital satisfaction.
Couples were interviewed by two interviewers who independently assessed each
participant’s
mate value and physical attractiveness. The results provided some support
for both hypothesized functions of self-esteem.
In another study, a colleague and I (Shackelford & Buss, 2000, Personality
and Individual Differences) tested the hypothesis that marital satisfaction
is a psychological state regulated by mechanisms that monitor
spousal cost-infliction. Three separate data sources were used. First, over
200 participants provided information on their personality and marital satisfaction.
Second, participants provided information on their spouse’s personality, mate guarding,
and susceptibility to infidelity. Third, couples were interviewed by two
interviewers, who subsequently provided independent ratings of each participant’s
personality. The results indicated that costs associated with spouse’s
personality, mate guarding, and susceptibility to infidelity negatively correlate
with participants’ marital satisfaction.
In addition to the above papers, I have published the results of my research
on the psychology of marital relationships in scholarly journals such as Journal
of Research in Personality (Buss & Shackelford, 1997), Journal of
Social and Personal Relationships (Shackelford & Buss, 1997), Journal
of Family Psychology (Shackelford & Buss, 1997), and Journal of
Personality and Social Psychology (Buss & Shackelford, 1997).
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Research
grants and contracts
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