Table 1.

Review results sorted by year of publication.

References	Terms used	Theoretical framework	Physiological measures	Setting	Clinical/psychological construct	Interpersonal physiology measure	Results
Di Mascio et al., 1955	“Sociophysiology” “Concordance” “Discordance”	Unspecified	HR	3 dyads, unspecified sessions: individual psychoanalytic psychotherapy	n/a	Correlation	Two examples, of HR “concordance” and “discordance,” were reported.
Coleman et al., 1956	“Physiological relationship”	Counter-transference, empathy (Reich, 1951)	HR	1 dyad, 44 sessions: individual psychoanalytic psychotherapy	Observer-rated patient's emotions (Custom categories: “Anxiety,” “Depression,” “Intra-punitive hostility,” “extra-punitive hostility”)	Graphical comparison of patient's and therapist's HR averaged within each emotion	Patient's and therapist's HR was similar for all emotions except “extra-punitive hostility” characterized by relatively higher therapist's HR.
				Subset of 17 sessions with both “Depression,” and “extra-punitive hostility” emotions	Therapist rated distractions or disturbing thoughts	Similarity of patient's and therapist's HR within the two emotions	Therapist distractions or disturbing thoughts were more frequent in low HR similarity sessions.
Dimascio et al., 1957	“Interpersonal physiology”	Unspecified	HRV	1 dyad, 12 sessions: individual psychoanalytic psychotherapy	Observer-rated salient segments of therapy: Bales Interaction Coding System (BICS; (Bales, 1951); Categories: “Tension,” “Tension release,” “Neutral,” “Disagreement,” “Antagonism”)	Graphical comparison of physiological activations (between-sessions within-subject correlations)	Patient's and therapist's HRV was similar in “Tension,” “Tension release,” and “Neutral” segments, but opposite in “Antagonism” segments.
McCarron and Appel, 1971	n/a	Information theory (Lennard and Bernstein, 1960)	SC	12 dyads, 1 session each: initial interviews; 1 dyad, 12 weekly sessions	Therapists' transcripts analysis with four classes of verbalizations: Reflection, Interpretation, Interrogation, and Confrontation (Pallone and Grande, 1965)	Graphical comparison of regressions results	A correspondence was found between the stimulus specificity of the therapists' verbalizations and the amplitude of patients' and therapists' SC.
Stanek et al., 1973	“Corresponding curve oscillations” “Co-ordination of the curves from doctor and patient”	Unspecified	HR	32 dyads, 1 session each: initial interviews of psychoanalytic treatment with cardiophobic patients	n/a	Graphical comparison of physiological activity in time	Sections of concordant and discordant patterns of HR reactions were reported as related to the content of the conversations.
Robinson et al., 1982	“Matching of phasic responding” “Phasic measure of affect” “Physiological response correlation”	Empathy	ST, SC	21 dyads, 2 sessions each: 30' psychological counseling (only the second session was analyzed)	Patients' perceived empathy: Barrett-Lennard Relationship Inventory's Empathic Understanding Subscale (EUS; Barrett-Lennard, 1986)	Correlations (ST); manual peak matching (SC)	The number of patients' SC peaks matched within 7 seconds with a counselors' peak showed strong correlation with perceived empathy.
Marci and Riess, 2005	“Physiologic concordance”	Empathy	SC	1 dyad, 1 session: individual psychotherapy	n/a	Unspecified time series analysis	A high degree of SC concordance between patient and therapist was reported
Marci and Orr, 2006	“Psychophysiologic concordance”	Empathy	SC	20 dyads, 1 session each: brief semi-structured interview with the same clinician. Half of the interviews were conducted in an emotionally neutral way, while the other half with emotional distance	Interviewer gaze direction; patients' perceived empathy: EUS	Moving window correlations (ratio of positive/negative ones)	SC concordance and perceived empathy were significantly lower in the “emotional distance” dyads.
Marci et al., 2007	“Physiologic concordance”	Empathy	SC	20 dyads, 1 session each: already established psychodynamic psychotherapy	Patients' perceived empathy: EUS; Social-Emotional Ratings: BICS	Moving window correlations (ratio of positive/negative ones)	SC concordance correlated with perceived empathy; during moments of high vs. low SC concordance, there were significantly more positive social- emotional interactions for both patients and therapists.
Stratford et al., 2009, 2012*	“Neurophysiological correlates of therapeutic alliance” “Physiological concordance” “Skin conductance resonance” “Therapeutic index”	Alliance	EEG, SC	30 dyads, 6 sessions each: relational client-centered psychotherapy	n/a	Moving window correlations (ratio of positive/negative ones)	Trends of distinct brain areas' activity were observed during high SC concordance, and were observed to vary across sessions.
Kleinbub et al., 2012a,b; Messina et al., 2013*	“Physiological concordance”	Empathy	SC	39 dyads, 1 session each. Each of 13 volunteers had a 20-min clinical interview with 3 different interviewers with different levels of training (psychotherapists, psychologists, no training)	Patients' perceived empathy (EUS); Observer rated empathy: Empathic Understanding in Interpersonal Processes Scale (EUIP; Carkhuff, 1969)	Moving window correlations (ratio of positive/negative ones); Lag analysis	SC concordance correlated with perceived empathy but not with observer-rated empathy. The three levels of training predicted higher concordance at different lags.
Seikkula et al., 2015	“Embodied Attunement” “ANS synchrony”	Embodiment, intersubjectivity	SC, HRV	1 tetrad (1 couple and 2 therapists), 1 session: dialogical psychotherapy	n/a	Graphical comparison of physiological activity in time	The clinical micro-process corresponding to dyadic or triadic synchronization of SC and HRV were reported.
Karvonen et al., 2016	“Embodied synchrony” “EDA concordance”	Unspecified	SC	10 tetrads (1 couple and 2 therapists), 1 session each: dialogical psychotherapy	Alliance: Session Rating Scale (SRS; Duncan et al., 2003)	Pairwise moving window correlations (ratio of positive/negative ones); Lag analysis	Higher than random SC synchrony was found in at least one dyad of each tetrad, mostly with a lag up to 1 s. On average therapist-therapist dyads had the most synchrony, while patient-patient ones the least. A correlation between SC synchrony and SRS was reported.
Päivinen et al., 2016	“Sympathetic synchrony” “Physiologic synchrony”	Unspecified	SC	2 tetrads (1 couple and 2 therapists), 1 session each: dialogical psychotherapy	Blaming: qualitative text analysis	Graphical comparison of physiological activity in time	SC activation of each tetrad's participants during blaming episodes were qualitatively reported. In some occasion therapist showed opposite arousal in respect to patients.
Orsucci et al., 2016	“Synchronization”	Dynamic complex system	SC	1 dyad, 1 session: short term cognitive-based psychotherapy	Linguistic prosody	Principal Component Analysis; Markov Transition Matrix; Cross-Recurrence Quantification Analysis	Cluster analysis of SC and prosodic synchrony reveals stable states that may match specific clinical processes. Further nonlinear properties of the session data are reported.
Kykyri et al., 2017	“Embodied attunement”	Embodiment	SC	1 tetrad (1 couple and 2 therapists), 1 session: dialogical psychotherapy	Linguistic prosody	Correlations	Various patterns of SC synchrony were qualitatively reported.
Palmieri et al., in press	“Physiological synchronization”	Attachment; Dyad system model	SC	18 dyads, 1 session each: 20′ clinical interviews. Half of the interviewers received a secure attachment priming	Attachment priming (Mikulincer et al., 2001)	Moving window correlations (ratio of positive/negative ones); Lag analysis	The priming procedure increased SC synchrony at negative lags (i.e., therapist leading) but not overall.

For each study, only the details, methods, and results directly assessing IP were included in this table, not the full design. SC, skin conductance; ST, skin temperature; HR, heart rate; HRV, heart rate variability; EEG, electroencephalography.

^*Articles sharing both the same dataset and similar design were collapsed in a single row.