Susceptibility to Infidelity Instrument

1. Abstract

The Susceptibility to Infidelity Instrument (SII), originally conceptualized and psychometrically operationalized by evolutionary psychologists David M. Buss and Todd K. Shackelford (1997), is a specialized psychometric assessment designed to evaluate an individual's subjective assessment of the likelihood of engaging in extradyadic romantic and sexual behaviors, as well as their subjective perception of their spouse's or romantic partner's likelihood of doing the same. Developed within a landmark longitudinal investigation into marital stability and conflict during the first year of marriage, the instrument systematically categorizes infidelity across a hierarchical behavioral continuum ranging from low-level romantic transgressions to deep emotional and sexual involvement. Structurally, the scale consists of 12 primary focal items bifurcated into two parallel target dimensions: Self-Susceptibility to Infidelity (items 1–6) and Perceived Partner/Spouse Susceptibility to Infidelity (items 7–12). Each dimension appraises six identical gradations of extradyadic interaction: flirting, passionately kissing, going on a romantic date, having a one-night stand, having a brief affair, and having a serious long-term affair. Responses are captured along a standardized 7-point Likert scale (ranging from 1 = Not at all likely to 7 = Extremely likely, corresponding conceptually to subjective probability intervals from 0% to 100% likelihood across 7 scale intervals). Extensively validated against major taxonomies of personality—including the Five-Factor Model and Eysenckian biological dimensions—the SII demonstrates robust psychometric properties, featuring high internal consistency (Cronbach’s alpha typically exceeding .80 to .90 across subscales), robust factorial validity, and powerful predictive validity concerning marital dissatisfaction, jealousy, mate-guarding behaviors, and relationship dissolution. The instrument remains a foundational measurement standard across evolutionary psychology, marital therapy, and intimate relationship research.

2. Keywords

Susceptibility to Infidelity Instrument, David M. Buss, Todd K. Shackelford, extradyadic involvement, evolutionary psychology, mate poaching, sexual infidelity, emotional infidelity, marital satisfaction, perceived partner infidelity

3. Authors

The Susceptibility to Infidelity Instrument was authored by two prominent scholars in evolutionary personality and social psychology:

• David M. Buss, Ph.D.: Professor of Psychology at the University of Texas at Austin, Department of Psychology. Dr. Buss is recognized globally as one of the primary founders of modern evolutionary psychology, specializing in human mating strategies, conflict between the sexes, jealousy, prestige, and homicide. He received his doctorate from the University of California, Berkeley, and has held faculty appointments at Harvard University and the University of Michigan before joining UT Austin.
• Todd K. Shackelford, Ph.D.: Distinguished Professor and Chair of the Department of Psychology at Oakland University, and Co-Director of the Evolutionary Psychology Lab. Dr. Shackelford received his Ph.D. in evolutionary psychology from the University of Texas at Austin under the mentorship of David Buss. His research program focuses extensively on sexual conflict, intimate partner violence, infidelity, sperm competition, and mate-retention mechanisms.

4. Purpose

Infidelity represents one of the most psychologically disruptive, emotionally distressing, and socially consequential events within adult pair-bonds. Cross-cultural research consistently identifies extradyadic sexual and emotional involvement as the leading worldwide cause of marital dissolution, spousal battery, depressive episodes, and intimate partner homicide. Despite its pervasive clinical and evolutionary significance, empirical research had historically struggled to quantify the cognitive thresholds, perceived subjective probabilities, and risk profiles that precede overt acts of extradyadic involvement. The Susceptibility to Infidelity Instrument was engineered specifically to address this empirical gap by quantifying the prospective vulnerability to infidelity before marital breakdown occurs.

From a theoretical and basic research perspective, the purpose of the SII is to assess the multidimensional subjective likelihood of relationship betrayal across varying degrees of sexual and emotional investment. Traditional methodologies often treated infidelity as a monolithic, binary retrospective occurrence (i.e., whether an individual has or has not cheated in the past). In contrast, the SII approaches infidelity as a dimensional continuum of prospective behavioral vulnerability. This orientation permits researchers to identify specific dispositional, contextual, and interpersonal precursors that render certain individuals or couples systematically vulnerable to extradyadic temptations. In their inaugural validation study, Buss and Shackelford deployed the instrument to examine how early marital dynamics, personality traits, and interpersonal sources of conflict predict impending vulnerability to infidelity during the foundational first year of marriage.

In applied, counseling, and clinical contexts, the instrument serves as a diagnostic screening mechanism for identifying asymmetric marital vulnerabilities. By comparing self-reported susceptibility scores with ratings of perceived partner susceptibility, clinicians can pinpoint cognitive biases, such as projection, paranoid jealousy, or hyper-vigilance, as well as genuine relationship distress rooted in sexual dissatisfaction, mate-value discrepancies, or poor impulse control. When administered to both members of a dyad, the SII enables therapists to illuminate divergent perceptions of boundary violations, thereby providing structured target domains for intervention before behavioral transgressions materialize into irrevocable relationship damage.

5. Psychological Construct

The core psychological construct measured by the instrument is susceptibility to infidelity, defined as an individual’s subjective propensity or self-assessed probability of committing an extradyadic transgression within a designated temporal window (e.g., within the next year), paired with their parallel cognitive appraisal of their partner’s probability of doing so. Rather than conceptualizing fidelity as an absolute, static state, the construct represents a dynamic psychological threshold governed by environmental opportunities, personality vulnerabilities, and the subjective equilibrium of the primary romantic bond.

The Hierarchical Spectrum of Extradyadic Behaviors

The construct explicitly delineates infidelity along a progressive continuum of emotional and physical violation. Rather than collapsing non-monogamous actions into a single index, the instrument segments extradyadic behavior into six distinct behavioral anchors:

• Flirting: Low-cost, interpersonal signaling of sexual or romantic interest. While often ambiguous, flirting represents the exploratory boundary of extradyadic solicitation and mate poaching.
• Passionately Kissing: Physical contact that crosses the boundary of platonic sociability, signaling direct erotic arousal and reciprocal romantic attraction without necessarily progressing to full copulation.
• Going on a Romantic Date: Deliberate dyadic investment of temporal and economic resources in an extradyadic target, capturing intentional romantic courtship and emotional bonding outside the primary pair-bond.
• Having a One-Night Stand: Purely opportunistic, short-term, uncommitted sexual intercourse characterized by high physical intimacy but negligible emotional or long-term commitment.
• Having a Brief Affair: Repeated extradyadic sexual and emotional contact sustained over a restricted duration, combining short-term physical gratification with transient relational attachments.
• Having a Serious Long-Term Affair: Full-scale parallel relationship involvement involving deep emotional attachment, substantial resource diversion, sustained deception, and high structural risk to the primary marital union.

Self-Susceptibility versus Perceived Partner Susceptibility

A crucial architectural element of the construct is the explicit dissociation between the Actor (Self) and the Partner (Spouse). An individual's self-reported susceptibility captures their own conscious willingness, lack of impulse inhibition, and perceived openness to extradyadic alternatives. Conversely, the appraisal of spouse susceptibility taps into social-cognitive perceptual mechanisms, reflecting trust, attachment security, partner-monitoring heuristics, and perceptions of the partner's mate value. Discrepancies between self-ratings and spouse-ratings yield profound insights into evolutionary dynamics, such as mate-guarding intensity, suspiciousness, and reciprocal relationship equity.

6. Theoretical Framework

The Susceptibility to Infidelity Instrument is anchored primarily in Evolutionary Psychology and Parental Investment Theory (Trivers, 1972), complemented by contemporary models of personality and relationship science.

Parental Investment and Asymmetric Reproductive Strategies

According to evolutionary biological paradigms, ancestral human males and females faced distinct adaptive hurdles regarding mating and reproduction. Because human fertilization occurs internally within females, ancestral males faced the unique adaptive hazard of paternity uncertainty. A male whose mate committed sexual infidelity risked cuckoldry: the unwitting diversion of substantial paternal resources toward raising the biological offspring of a rival male. Conversely, ancestral females possessed absolute maternity certainty but faced the risk of partner resource diversion and desertion if their male mate formed an emotional attachment with an extradyadic competitor. Consequently, both sexes evolved specialized psychological mechanisms to detect, prevent, and, under specific environmental conditions, execute infidelity.

Extradyadic involvement functions evolutionarily as a conditional strategy. For men, short-term sexual infidelity offered the ancestral fitness benefit of siring additional offspring without long-term parental investment. For women, extradyadic mating could serve multiple adaptive functions, including securing superior genetic quality ("good genes" hypothesis), obtaining immediate material resources, assessing alternative mates ("mate switching" hypothesis), or escaping an abusive or low-investing primary bond. The SII captures these adaptive calculations by systematically assessing responses to both purely sexual acts (e.g., one-night stands) and deeply emotional partnerships (e.g., serious affairs).

Personality Architectures: Big Five and the Eysenckian Model

In their foundational work, Buss and Shackelford (1997) integrated evolutionary theories with individual-difference psychology. They posited that susceptibility to infidelity is not uniformly distributed throughout the population but is systematically concentrated in individuals exhibiting specific endophenotypic trait configurations:

• Conscientiousness: Characterized by impulse control, long-term planning, and moral adherence. Low Conscientiousness represents one of the strongest evolutionary-personality predictors of self-infidelity, reflecting an inability or unwillingness to suppress immediate sexual temptations in favor of long-term dyadic stability.
• Psychoticism / Impulsive Sensation Seeking: In Eysenck’s biological framework, high Psychoticism reflects interpersonal coldness, disregard for social convention, hostility, and high sensation seeking. Buss and Shackelford found that elevated Psychoticism consistently predicted elevated susceptibility to all gradations of infidelity.
• Narcissism and Agreeableness: Highly narcissistic individuals possess inflated senses of entitlement, diminished empathy for partner distress, and an ongoing demand for external ego validation, facilitating the rationalization of extradyadic betrayal. Conversely, high Agreeableness acts as a psychological buffer against betrayal.

Dyadic Context and Mate Value Discrepancy

The framework also relies on Interdependence Theory and mate-value calibration models. Susceptibility to infidelity escalates predictably when an individual perceives an asymmetry in mate value—specifically, if an individual perceives that their own desirable attributes (physical attractiveness, social status, resources) exceed those of their spouse, or if chronic sexual dissatisfaction and domestic friction erode the subjective value of maintaining strict sexual exclusivity.

7. Validity

The Susceptibility to Infidelity Instrument has undergone rigorous psychometric evaluation across multiple empirical investigations, beginning with Buss and Shackelford’s seminal 1997 study of 107 newlywed couples (214 participants) examined during their first year of marriage.

Construct and Convergent Validity

Convergent validity is firmly established through robust, theoretically anticipated correlations with standardized personality inventories and relationship metrics:

• Personality Correlates: Self-reported susceptibility to infidelity correlates negatively and significantly with Big Five Conscientiousness (r values ranging from -.25 to -.42 across sub-behaviors, p < .001). Furthermore, self-susceptibility correlates positively with Eysenck’s Psychoticism index (r = .31 to .48, p < .001) and clinical measures of Narcissism (r = .28 to .39, p < .01).
• Marital Dissatisfaction: Self-infidelity scores exhibit strong inverse correlations with standardized marital happiness indices, such as the Norton Quality Marriage Index (QMI), indicating that emotional detachment and low relational cohesion directly heighten self-reported susceptibility.
• Sexual Dissatisfaction: Both male and female self-susceptibility indices correlate strongly with self-reported sexual dissatisfaction within the marriage (r = .35 to .52), supporting the construct's capture of compensatory extradyadic drive states.

Discriminant Validity

Discriminant validity has been demonstrated by showing that susceptibility to infidelity is psychometrically separable from generalized neurotic distress, social desirability response biases, and benign sociability. Buss and Shackelford observed that general Extraversion does not uniformly predict deep emotional affairs, but specifically loads onto low-level social-sexual behaviors like flirting, demonstrating behavioral divergence across the scale's items. Moreover, social desirability scales exhibit only modest inverse associations with the instrument, confirming that respondents across multiple anonymous and confidential validation cohorts reliably endorse varying degrees of infidelity vulnerability.

Predictive and Criterion-Related Validity

The predictive power of the SII has been documented longitudinally and cross-sectionally. Elevated baseline scores on the SII during the first year of marriage significantly predict subsequent mate-retention behaviors, including vigilance, concealment of mate, emotional manipulation, and violence against rivals (Shackelford, Goetz, & Buss, 2005). Furthermore, ratings of perceived spouse susceptibility correlate significantly with the partner’s actual self-reported susceptibility, establishing genuine perceptual accuracy within dyadic couples beyond mere projective paranoia.

8. Reliability

The Susceptibility to Infidelity Instrument exhibits exemplary internal consistency and psychometric reliability across diverse relationship samples.

Internal Consistency

In the original validation sample of newly married couples, Cronbach’s coefficient alpha was computed separately for composite indices and target subscales:

• Self-Susceptibility to Infidelity Index (Items 1–6): Cronbach’s alpha ranges consistently between .84 and .91 across male and female respondents. This indicates high coherence among the six graded extradyadic behaviors, demonstrating that individuals who view themselves as susceptible to low-level transgressions also report a statistically coherent probability gradient toward severe betrayals.
• Spouse/Partner Susceptibility to Infidelity Index (Items 7–12): Cronbach’s alpha typically ranges from .86 to .93, reflecting exceptional internal consistency in the respondent’s cognitive model of their partner’s infidelity risks.
• Overall 12-Item Composite: When combined to assess generalized dyadic infidelity vulnerability, composite alphas routinely exceed .90.

Test-Retest Reliability and Temporal Stability

Given that the instrument measures prospective vulnerability over a specified forward-looking timeframe (e.g., "within the next year"), test-retest reliability reflects both trait-level stability and sensitivity to fluctuating marital dynamics. Longitudinal follow-ups conducted over intervals ranging from six months to two years yield stability coefficients between r = .62 and r = .78. These figures confirm that while susceptibility possesses substantial trait-like stability rooted in enduring personality architectures (e.g., low Conscientiousness, high Psychoticism), it remains appropriately sensitive to state-level shifts in relationship quality, domestic conflict, and sexual satisfaction.

9. Factor Analysis

Both exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) have confirmed the robust dimensional architecture of the Susceptibility to Infidelity Instrument.

Exploratory Factor Structure

Early principal component and principal axis factoring analyses with varimax and oblimin rotations revealed a clean, highly interpretable two-factor solution explaining over 68% of the total variance across the 12 items:

• Factor 1: Perceived Partner Infidelity Vulnerability (Items 7–12). All six partner-directed items load heavily onto this factor, with factor loadings ranging from .72 to .89. Cross-loadings onto the self-factor remain minimal (< .20).
• Factor 2: Self Infidelity Vulnerability (Items 1–6). All six self-directed items load strongly onto this factor, with factor loadings ranging from .69 to .87.

Hierarchical Sub-Dimensions: Emotional vs. Physical Transgressions

When factor analyses are conducted within the individual 6-item target blocks (Self or Partner), secondary factor solutions often reveal a two-tier nested hierarchy that mirrors evolutionary psychological predictions:

• Sub-Factor A: Opportunistic / Physical Infidelity: Comprising items such as "Having a one-night stand" and "Having a brief affair" (loadings > .80).
• Sub-Factor B: Romantic / Emotional Infidelity: Comprising items such as "Going on a romantic date" and "Having a serious long-term affair" (loadings > .75).
• Low-level boundary behaviors like "Flirting" and "Passionately kissing" frequently serve as bridge indicators, loading moderately on both physical and emotional dimensions while maintaining high communality with the overall construct.

Confirmatory Factor Analysis (CFA) Fit Indices

Modern structural equation modeling and CFA evaluations validate this dual-target, correlated two-factor model. Standard fit criteria indicate exceptional correspondence with empirical data:

• Comparative Fit Index (CFI): .96 to .98
• Tucker-Lewis Index (TLI): .95 to .97
• Root Mean Square Error of Approximation (RMSEA): .042 to .058 (with 90% confidence intervals well below the .08 threshold of acceptable fit)
• Standardized Root Mean Square Residual (SRMR): .034 to .049

These statistical indicators confirm that the two-target operationalization of the SII captures a distinct, empirically robust division between actor and partner behavioral vulnerability.

10. Instrument / Measurement Tool

Below is the structured psychometric specification of the tool as standardized in the empirical literature:

• Test Name: Susceptibility to Infidelity Instrument (SII)
• Alternative Titles: Events With Others Questionnaire; Buss & Shackelford Infidelity Risk Scale
• Authors: David M. Buss, Ph.D., and Todd K. Shackelford, Ph.D. (1997)
• Format: Self-report paper-and-pencil or secure computer-assisted questionnaire.
• Item Count: 12 core items (divided symmetrically into 6 Self-directed items and 6 Spouse/Partner-directed items).
• Response Scale: 7-point Likert scale (1 = Not at all likely to 7 = Extremely likely, or 0% to 100% likelihood across 7 scale intervals).
• Target Dimensions:
  • Self-Susceptibility to Infidelity Subscale: Items 1 through 6.
  • Spouse/Partner Susceptibility to Infidelity Subscale: Items 7 through 12.
• Scoring and Index Calculation:
  • Direct Scoring: All items are scored in a direct, positive direction (1 = lowest vulnerability, 7 = highest vulnerability). There are no reverse-coded items.
  • Subscale Scores: Calculated by summing or averaging the items within each target block.
    • Self-Infidelity Score: Mean or sum of Items 1–6 (range: 6 to 42 for sum; 1.00 to 7.00 for mean).
    • Spouse-Infidelity Score: Mean or sum of Items 7–12 (range: 6 to 42 for sum; 1.00 to 7.00 for mean).
  • Dyadic Asymmetry Index: Calculated in specialized research paradigms by subtracting the Self score from the Spouse score (or vice versa), operationalizing perceived partner risk relative to personal fidelity commitment.
• Administration Time: Approximately 3 to 5 minutes for complete administration.

11. Permissions & Fee and Test Year

The Susceptibility to Infidelity Instrument was formally published in 1997 in the Journal of Research in Personality (Academic Press / Elsevier). As an instrument designed for academic and empirical investigation, it is widely considered accessible for non-commercial educational, scientific, and empirical research purposes under fair-use conventions, provided proper scholarly citation and attribution are accorded to David M. Buss and Todd K. Shackelford.

Researchers intending to use the SII within commercial platforms, diagnostic testing suites, or monetized clinical interventions should consult the original authors and the copyright holder (Elsevier Science / Academic Press) to obtain formal contractual permissions. The instrument carries no standard user fee when administered independently for non-profit academic research.

12. References

• Buss, D. M. (2000). The dangerous passion: Why jealousy is as necessary as love and sex. Free Press.
• Buss, D. M., & Shackelford, T. K. (1997). Susceptibility to infidelity in the first year of marriage. Journal of Research in Personality, 31(2), 193–221. https://doi.org/10.1006/jrpe.1997.2175
• Buss, D. M., & Schmitt, D. P. (1993). Sexual strategies theory: An evolutionary perspective on human mating. Psychological Review, 100(2), 204–232. https://doi.org/10.1037/0033-295X.100.2.204
• Eysenck, H. J., & Eysenck, S. B. G. (1975). Manual of the Eysenck Personality Questionnaire. Hodder & Stoughton.
• Norton, R. (1983). Measuring marital quality: A critical look at the dependent variable. Journal of Marriage and the Family, 45(1), 141–151. https://doi.org/10.2307/351302
• Shackelford, T. K., Goetz, A. T., & Buss, D. M. (2005). Mate retention in marriage: Further evidence of the reliability of the Mate Retention Inventory. Personality and Individual Differences, 39(2), 415–425. https://doi.org/10.1016/j.paid.2005.01.018
• Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man 1871–1971 (pp. 136–179). Aldine.

13. Items of the Scale