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Psychophysiological Effects of Equine-Assisted Psychotherapy in Trauma Recovery: A Review of Current Evidence 
Brittin Palmer
Ph.D. Student, Department of Applied Psychophysiology
College of Integrative Medicine and Health Sciences
Saybrook University
Published online: December 2025
  © The Author(s) 
Abstract
Background: Equine-assisted psychotherapy (EAP) has gained increasing attention as a clinically grounded intervention for trauma recovery, yet the psychophysiological mechanisms underlying its effects remain insufficiently understood. This paper reviews current evidence on EAP with a focus on outcomes relevant to stress physiology, including changes in cortisol, heart-rate variability (HRV), and oxytocin, along with associated psychological and behavioral improvements.
Methods: Research in adolescents, adults, and veteran populations suggests that EAP may support emotion regulation, interpersonal functioning, and reductions in trauma-related symptoms. Preliminary biomarker data indicate patterns consistent with improved autonomic and HPA-axis regulation, particularly decreases in cortisol and increases in oxytocin during equine-facilitated psychotherapy for human participants. In addition, welfare-focused studies indicate that horses participating in EAP and related equine-assisted activities do not exhibit significant increases in physiological or behavioral stress markers, suggesting that well-structured programs can be conducted ethically.
Results: Together, these findings highlight the potential of EAP as a complementary, experiential modality for trauma recovery, while underscoring the need for more rigorous examination of underlying biological mechanisms. Methodological limitations, such as small samples, varied intervention protocols, and scarce physiological measurements, hinder definitive conclusions. 
Conclusions: This review synthesizes empirical findings and conceptual models to evaluate EAP’s role within trauma-informed psychophysiology and identifies directions for future physiology-informed research.
Keywords: Equine-assisted psychotherapy (EAP), trauma recovery, emotional regulation, interpersonal functioning


Introduction
Equine-assisted psychotherapy (EAP) is a specialized clinical modality in which a licensed mental health provider and an equine specialist facilitate psychotherapeutic processes involving horses. Unlike broader equine-assisted activities or therapeutic riding, EAP is explicitly grounded in psychological theory, employs structured therapeutic goals, and is delivered as a mental health treatment. This distinction is essential, as the research literature often conflates EAP with non-clinical equine programs, making it challenging to interpret findings or evaluate mechanisms of change.
Interest in EAP has grown within trauma-informed care because of its potential to engage core domains disrupted in trauma, including autonomic regulation, attachment, and somatic awareness. Trauma frequently involves dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis, heightened sympathetic arousal, and reduced parasympathetic tone. These physiological patterns contribute to hypervigilance, emotional reactivity, and difficulties accessing cues of safety, all of which can complicate traditional talk-based therapy. The experiential, relational, and embodied nature of EAP may offer an avenue for addressing these psychophysiological disruptions more directly.
The purpose of this paper is to review empirical evidence on the psychophysiological and psychological outcomes of EAP, evaluate the strength of current findings, and situate EAP within trauma-recovery frameworks informed by physiological psychology. Emphasis is placed on studies that measure biological outcomes such as cortisol, oxytocin, and heart-rate variability (HRV), as these are central to understanding how therapeutic processes may influence the nervous system.

Background and Conceptual Foundations
Equine-assisted psychotherapy integrates experiential, relational, and somatic elements that align closely with models of trauma recovery. Lee, Dakin, and McLure (2016) describe EAP as a psychotherapy delivered collaboratively by a mental health clinician and an equine specialist, with the horse serving as a responsive partner in the therapeutic process. Because horses are highly attuned to nonverbal communication and emotional states, their reactions offer immediate, embodied feedback that can help clients recognize and regulate internal cues.
From a psychophysiological perspective, trauma is associated with disruptions in autonomic balance, including sympathetic hyperarousal, low heart-rate variability, and impaired engagement of the parasympathetic nervous system. Chronic activation of the HPA axis can contribute to persistent elevations or instability in cortisol levels, reducing an individual’s capacity for calm, flexible responding. EAP may intersect with these systems through mechanisms such as co-regulation with the horse, activation of affiliative neurobiology, and the multisensory grounding inherent in equine interaction. These theoretical pathways provide a foundation for examining whether EAP can facilitate measurable biological changes relevant to trauma recovery.
Psychophysiological Mechanisms Relevant to Trauma and EAP
Trauma-related symptoms often emerge from the body’s stress-response systems rather than purely cognitive or emotional processes. Dysregulated ANS activity, including low HRV and heightened sympathetic tone, contributes to hypervigilance and difficulty maintaining emotional stability. Likewise, disruptions in the HPA axis may lead to altered cortisol secretion patterns, affecting energy, alertness, and stress sensitivity. Mechanisms involving oxytocin, a neuropeptide linked to bonding and social regulation, are also relevant, as trauma may reduce access to feelings of safety and connection.
EAP may influence these systems in several ways. Interacting with horses can evoke social engagement cues that support parasympathetic activation, potentially increasing HRV. Positive human–animal interactions have been associated with oxytocin release, which may enhance trust, bonding, and physiological calming. Additionally, the rhythmic, sensory-rich, and relational qualities of equine engagement may help shift autonomic patterns from defensive to regulated states. While these mechanisms are theoretical, they provide a compelling rationale for investigating the physiological effects of EAP in trauma contexts.
Empirical Findings
The empirical literature on EAP demonstrates promising psychological outcomes but remains limited in methodological rigor and consistency. In their review of 24 EAP studies, Lee et al. (2016) reported improvements in emotional regulation, interpersonal functioning, and behavioral outcomes—particularly among adolescents. However, they emphasized that most studies relied on self-report measures, lacked standardized intervention protocols, and did not include physiological data, making it difficult to identify mechanisms underlying observed benefits.
In adult clinical populations, several studies document meaningful symptom improvements. Burton et al. (2019) found reductions in PTSD symptoms and increases in resilience among veterans participating in a six-week EAP program, although statistical power was limited due to small sample sizes. Machová et al. (2023) reported significant gains in global functioning, mood, and quality of life among individuals in residential substance-use treatment receiving equine-facilitated psychotherapy compared with treatment-as-usual controls. These outcomes highlight the potential therapeutic value of EAP but do not clarify whether physiological changes accompany psychological improvements.
A small but growing number of studies have measured psychophysiological variables directly. McDuffee et al. (2024) conducted the most comprehensive investigation to date, examining salivary cortisol, oxytocin, and HRV in veterans with PTSD participating in an eight-week equine-facilitated psychotherapy program. Their findings indicated decreases in cortisol and increases in oxytocin across sessions, alongside improved mood and reduced anxiety—patterns consistent with enhanced autonomic and HPA-axis regulation. Importantly, their study also monitored horses’ physiological responses and found no evidence of stress, supporting the ethical feasibility of EAP for equine partners.
Given the variability in research designs, populations, and outcome measures, it is useful to summarize key EAP studies that address psychological or physiological outcomes relevant to trauma recovery. Table 1 provides an overview of representative empirical investigations, including study designs, participant characteristics, psychophysiological measures, main findings, and limitations. This summary highlights both the strengths and the methodological gaps in the current evidence base, particularly regarding physiological mechanisms.







Table 1. Summary of Representative Studies Examining Psychological and Psychophysiological Outcomes in Equine-Assisted Psychotherapy (EAP)
	Study
	Population & Sample
	Design & Intervention
	Psychophysiological Measures
	Key Findings
	Limitations

	McDuffee et. al. (2024)
	Veterans with PTSD; repeated measures across 8-week program
	Equine-facilitated psychotherapy delivered by clinician + equine specialist
	Salivary cortisol; oxytocin; HRV (humans); cortisol & HRV (horses)
	Decreased cortisol; increased oxytocin; HRV shifts consistent with improved regulation; improved mood/anxiety; horses showed neutral or positive physiological responses
	Small sample; no randomized control group; modest physiological changes; limited generalizability

	Burton et. al. (2019)
	Veterans with PTSD (EAP n ≈ 15; comparison n ≈15
	Six week EAP vs. treatment as usual
	None
	Within-group reductions in PTSD symptoms and improvement in resilience
	Small sample; limited statistical power; no physiological measures

	Machová et al. (2023)
	Adults in residential substance-use treatment (EFPP n=39; control n=18)
	Equine-facilitated psychotherapy integrated into inpatient program
	None
	Improvements in functioning, mood, and quality of life compared to controls
	No physiological assessment; not trauma-specific

	Matlock et al. (2025)
	18 horses participating in an EAL program (n = 11 trauma-history youth; n = 7 young adults with developmental delays)
	Pilot study comparing horse physiological and behavioral stress responses across two EAL participant groups
	Salivary cortisol, eye temperature, behavioral stress indicators
	No significant differences in cortisol, eye temperature, or behavioral stress markers between trauma-history youth sessions and control sessions; all physiological values remained within normal ranges.
	Small sample size; pilot design; limited generalizability; EAL not equivalent to psychotherapy.


	Neff (2013)
	8 Horses participating in EAP programs 
	Observational equine welfare study
	Horse cortisol; heart rate; respiration
	No physiological indicators of stress in horses during sessions
	No human data; small exploratory design

	Lee et al. (2016)
	Narrative review of 24 EAP psychotherapy studies
	Synthesis of psychotherapy-focused EAP programs
	Few studies included physiological measures
	Improved emotional, social, and behavioral functioning in EAP
	Heterogeneous designs; minimal physiological evidence



As summarized in Table 1, the strongest physiological evidence for EAP comes from McDuffee et al. (2024), who demonstrated measurable changes in cortisol, oxytocin, and HRV alongside improvements in mood and anxiety among veterans with PTSD. These findings support the hypothesis that EAP may influence stress-response systems relevant to trauma recovery. In contrast, studies by Burton et al. (2019) and Machová et al. (2023) showed meaningful psychological improvements but did not assess biological markers, leaving open the question of whether their clinical effects were supported by physiological shifts. Lee et al. (2016) further emphasized that early EAP research rarely incorporated objective physiological measurement, underscoring an ongoing gap between theoretical expectations and empirical evaluation. Neff (2013) conducted a study of eight horses engaged in equine-facilitated activities across three seasons—spring, summer, and fall—measuring both physiological and behavioral indicators of stress. The horses did not exhibit significant increases in physiological or behavioral stress markers during or immediately after sessions, and by the end of the fall season their measured stress indicators were the lowest. This seasonal pattern underscored the influence of circadian rhythms and consistent daily routines on equine stress regulation—an important consideration when evaluating animal welfare in EAP. Another study examining the welfare of equine partners in EAP and EAL evaluated whether working with trauma-exposed youth produced greater stress responses in horses than working with individuals with other disabilities. Matlock et al. (2025) assessed 18 horses using salivary cortisol, eye temperature, and behavioral stress indicators and found no significant differences between the trauma-history and comparison groups. All physiological values remained within normal ranges, indicating that participation in EAL sessions with trauma survivors did not elevate horses’ stress levels.
Limitations in the Current Evidence Base
Although EAP shows promise, several limitations constrain the strength of current conclusions. One central issue is inconsistent terminology: many studies use the labels “equine-assisted psychotherapy,” “equine therapy,” or “equine-assisted activities” interchangeably, despite significant differences in treatment structure, clinician involvement, and therapeutic goals. This variation makes cross-study comparison challenging and complicates the identification of which specific components of EAP drive therapeutic change.
Methodological weaknesses also limit interpretation. Most EAP studies rely on small samples and lack randomization, active control groups, or standardized treatment protocols. For example, Burton et al. (2019) reported psychological improvements among veterans receiving EAP, but the limited sample size reduced statistical power and hindered group comparisons. Furthermore, studies often rely heavily on self-report measures, which may not fully capture physiological aspects of trauma recovery.
Finally, few studies include psychophysiological measures such as cortisol, HRV, or oxytocin. While McDuffee et al. (2024) demonstrated promising biomarker changes, replication with larger samples and more rigorous controls is needed. Standardized protocols for timing, collection, and interpretation of physiological data would enhance reliability and facilitate integration of biological and psychological outcomes.
Future Directions for Research
Building a stronger evidence base for EAP will require addressing methodological gaps and expanding the use of physiological measures. Future research should incorporate larger samples, randomization where feasible, and well-defined control conditions to improve internal validity. Clear articulation of EAP protocols—including session structure, therapeutic goals, clinician qualifications, and the role of the equine specialist—would enable more precise replication and facilitate meaningful comparison across studies.
Integrating psychophysiological measurements is especially important for understanding whether and how EAP influences trauma-related biological systems. Biomarkers such as cortisol, HRV, respiratory sinus arrhythmia, and oxytocin can provide objective insights into shifts in autonomic and HPA-axis functioning. Standardizing collection times, environmental conditions, and baseline measures would improve reliability. In addition, analyzing the physiological responses of both humans and horses could illuminate patterns of co-regulation, a concept central to many theoretical models of EAP.
Implications for Clinical Practice
Despite its limitations, the existing evidence suggests several implications for trauma-informed clinicians incorporating EAP. First, EAP appears to support core processes involved in trauma recovery, such as emotion regulation, interoceptive awareness, and relational engagement. Because trauma often manifests in physiological patterns of hyperarousal, shutdown, or dysregulated stress responses, EAP’s embodied, experiential nature may offer access to regulation pathways that traditional talk therapy alone may not fully engage.
Second, clinicians may benefit from considering simple physiological tracking tools—such as HRV monitors or structured self-report regulation check-ins—to enhance treatment planning and client insight. While biological measures are not necessary for effective practice, awareness of how EAP may interface with the nervous system can deepen case conceptualization and help tailor interventions.
Third, equine welfare remains central to effective EAP. Studies that monitor horses’ physiological responses, including Neff (2013) and McDuffee et al. (2024), indicate that well-implemented EAP does not impose undue stress on equine partners. Ongoing attention to the horse’s physical and emotional state supports ethical practice and contributes to a safe, mutually regulated therapeutic environment.
Conclusion
Equine-assisted psychotherapy shows promise as a trauma-informed intervention capable of influencing both psychological and physiological domains of functioning. While research consistently demonstrates improvements in emotional regulation, interpersonal functioning, and trauma-related symptoms, only a limited number of studies have examined underlying biological mechanisms. Preliminary evidence suggests that EAP may positively influence stress-related physiology, including cortisol, oxytocin, and HRV, supporting models of autonomic and affective regulation in people. However, methodological inconsistencies, small sample sizes, and limited biomarker data highlight the need for more rigorous research. EAP offers an experiential, relational approach that may meaningfully complement traditional therapies for trauma recovery, particularly when integrated within frameworks informed by psychophysiological principles. Available equine-welfare data also show that horses do not exhibit increased physiological stress during properly structured sessions, supporting the ethical viability of EAP.




























    References

Burton, L. E., Qeadan, F., & Burge, M. R. (2019). Efficacy of equine-assisted psychotherapy in veterans with posttraumatic stress disorder. Journal of Integrative Medicine, 17(1), 14–19.https://www.sciencedirect.com/science/article/abs/pii/S2095496418301158?via%3Dihub. 
Lee, P.-T., Dakin, E., & McLure, M. (2016). Narrative synthesis of equine-assisted psychotherapy literature: Current knowledge and future research directions. Health & Social Care in the Community, 24(3), 225–246. https://onlinelibrary.wiley.com/doi/10.1111/hsc.12201
Machová, K., Svoboda, M., Mach, J., Št’astná, L., & Papežová, H. (2023). An evaluation of the effect of equine-facilitated psychotherapy on patients with substance use disorders: A naturalistic controlled trial. PLOS ONE, 18(6), e0286867. https://doi.org/10.1371/journal.pone.0286867
Matlock, S. K., Singh, A., Grandin, T., Merritt, T., Nett, T., Reega, S. J., & Peters, B. C. (2025). Behavioral and physiological indicators of stress in horses during an equine-assisted learning program for youth with a history of trauma. Translational animal science, 9, txaf027. https://pubmed.ncbi.nlm.nih.gov/40191691/
McDuffee, L. A., Montelpare, W. J., & LeBlanc, C. (2024). Psychophysiological effects of equine-facilitated psychotherapy on veterans with PTSD and their horse partners. Journal of Military, Veteran and Family Health, 10(3), 135–147. https://psycnet.apa.org/record/2025-01086-012
Neff, B. (2013). The effects of equine-assisted psychotherapy on stress of the horse: Measuring cortisol, pulse, respiration, and behaviors before and after clinical sessions (Unpublished undergraduate thesis). Maryville College. https://www.maryvillecollege.edu/wp-content/uploads/Faculty/NaturalSciences/dcrain/undergraduate-research/Neff2013.pdf. 


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