Alcohol and marijuana are two of the most widely used substances in the U.S., with rates of marijuana use and its co-use with alcohol increasing (McCabe et al., 2021). Alcohol and marijuana co-use can refer to using both substances during the same general period of time (e.g., past month) but not necessarily at the same time (concurrent alcohol and marijuana [CAM] use), or using both substances at the same time such that their effects overlap (simultaneous alcohol and marijuana [SAM] use; Midanik et al., 2008). There is a plethora of research linking alcohol use with physical and psychological intimate partner aggression (IPA) perpetration (Leonard & Quigley, 2017); however, far less research has shown that marijuana use is positively associated with IPA (Shorey et al., 2018, Testa et al., 2018). And it is not known how SAM use is associated with IPA perpetration specifically or how the effects of SAM use on IPA perpetration may differ from those of CAM or alcohol use alone. Because SAM use is associated with heavier drinking, more alcohol-related problems, and more subjective impairment than CAM use or alcohol use only (Lee et al., 2022, Linden-Carmichael et al., 2020, Subbaraman and Kerr, 2015), understanding the impact of alcohol and marijuana co-use patterns on IPA perpetration is imperative for future prevention and intervention efforts.

Rates of IPA perpetration are high among couples in the US, with between 20-37% reporting physical aggression (i.e., a physical attack such as hitting or pushing; Bell et al., 2007; Silverman et al., 2001) and 70-90% reporting psychological aggression (e.g., manipulation of a partner via intimidation or verbal attacks; Neufeld et al., 1999, Shorey et al., 2008). Rates of IPA tend to increase through young adulthood, plateau, and then decrease over time as individuals age (O’Leary, 1999). Additionally, although women tend to sustain more injuries as a result of IPA compared to men (Archer, 2000), numerous studies have found no difference in rates of physical and psychological aggression perpetration for both men and women (Shorey et al., 2008, Stappenbeck and Fromme, 2014). However, rates of IPA among sexual minority individuals are equal to or greater than rates observed among heterosexual individuals, perhaps due to factors associated with minority stress (e.g., internalized homonegativity; Edwards et al. 2015).

A prevailing unified theory of IPA, the I3 model, posits that the likelihood of IPA perpetration is increased when instigation (factors that create urges to aggress) and impellance (factors that strengthen urges to aggression) are high and inhibition (factors that promote resistance toward aggression) is low (Finkel, 2007; Eckhardt et al., 2015). COVID-19 stress has been shown to be a critical instigator of IPA perpetration (Hammett et al., 2022, Parrott et al., 2022). Additionally, inhibitory capabilities are lowered by disinhibitory factors such as alcohol use. Indeed, more frequent drinking and alcohol-related problems are major contributing causes of both physical and psychological IPA (Leonard, 2005; Leonard & Quigley, 2017; Shorey et al., 2011), increasing not only their occurrence but also their severity (Testa et al., 2003). Alcohol Myopia Theory purports that alcohol’s acute physiological effects create a myopic experience in which reduced cognitive capacity narrows one’s attentional focus to the most salient cues in their environment (Steele & Josephs, 1990). This narrowed attention is thought to increase one’s focus on salient instigatory cues that could propel an individual to act aggressively. Meta-analyses estimate the effect size of alcohol on IPA perpetration to be moderate (Cafferky et al., 2018; Foran & O’Leary, 2008), indicating that alcohol use is neither a necessary nor sufficient criterion for IPA perpetration (Parrott & Eckhardt, 2018) and that alcohol use may be interrelated with other risky behaviors to facilitate IPA (Leonard & Quigley, 2017).

Findings with regards to marijuana’s association with IPA are inconsistent, with some studies finding a positive association between marijuana use and IPA (Cunradi et al., 2015; 2020; Flanagan et al., 2020, Shorey et al., 2018, Testa et al., 2018), other studies finding no association (Crane et al., 2014, Shorey et al., 2014, Testa et al., 2003), and at least two studies finding a negative association (Smith et al., 2014, Stuart et al., 2013). Moreover, laboratory studies of the effects of delta-9-tetrahydrocannabinol (THC) on aggression found that THC was associated with increased aggression at lower but not higher doses (Myerscough and Taylor, 1985, Taylor et al., 1976). These inconsistent associations with IPA may be due, in part, to marijuana’s anxiolytic effects (Bonn-Miller et al., 2007). However, marijuana’s acute effects are varied and may include physiological arousal, cognitive impairment, attentional bias for negatively-valenced visual stimuli, greater perceptions of hostility in others, and impaired inhibition (Ansell et al., 2015, Dahlgren et al., 2016, Metrik et al., 2012; 2015), factors associated with increased risk for IPA (Testa & Brown, 2015). Additionally, patterns of marijuana use, and particularly the co-use of marijuana and alcohol (e.g., CAM or SAM use), may help explain the inconsistent findings between marijuana use and IPA.

SAM use has been associated with heavier drinking compared to CAM and alcohol use only (Lee et al., 2020, Linden-Carmichael et al., 2019, Subbaraman and Kerr, 2015), and SAM users endorse more alcohol-related problems and negative consequences compared to those who use alcohol only (Lee et al., 2020, Linden-Carmichael et al., 2019). Of note, SAM use has been associated with disinhibition, including more difficulty concentrating, relative to alcohol and marijuana use alone (Lee et al., 2017). Moreover, data suggest that SAM use is associated with increased subjective intoxication and behavioral impairment relative to CAM use (Chait & Perry, 1994). Taken together, the increased disinhibition, subjective intoxication, and behavioral impairment associated with SAM use may increase one’s risk for IPA relative to CAM and alcohol use alone. However, to our knowledge, no peer-reviewed study has directly investigated the influence of SAM use on IPA perpetration. Because of the potential for increased IPA risk associated with SAM use, and the increase in marijuana as well as alcohol and marijuana co-use (McCabe et al., 2021), it is important to examine the association among alcohol and marijuana co-use patterns on IPA.