The sample comprised participants from the ALSPAC cohort (Figure 1). During Phase I enrolment, 14,541 pregnant mothers residing in the former Avon Health Authority in the south-west of England with expected dates of delivery between 1 April 1991 and 31 December 1992 were recruited. The total sample size is 15,454 pregnancies, of which 14,901 were alive at 1 year of age. ALSPAC Generation 2 (ALSPAC-G2) was set up to provide a unique multigenerational cohort and builds on the existing ALSPAC resource of originally recruited women and their partners (Generation 0; ALSPAC-G0) and their offspring (ALSPAC-G1) followed up for 26 years. Recruitment of the next generation ALSPAC-G2—the grandchildren of ALSPAC-G0 and children of ALSPAC-G1—began on 6th June 2012. Up to 30th June 2018, 810 ALSPAC-G2 participants from 548 families had been recruited. Over 70% of those invited to early- and late-pregnancy, second week of life, 6-, 12- and 24-month assessments attended, with attendance >60% for subsequent visits up to 7 years. Further details on the cohort profile, representativeness and phases of recruitment, including ALSPAC-G2, are described in four cohort-profile papers (53–56). ALSPAC study website www.bristol.ac.uk/alspac/ contains details of all the data that is available through a fully searchable data dictionary and variable search tool (http://www.bris.ac.uk/alspac/researchers/our-data/).

Informed consent for the use of data collected via questionnaires and clinics was obtained from participants following the recommendations of the ALSPAC Ethics and Law Committee at the time. Study data were collected and managed using Research Electronic Data Capture (REDCap), a secure web-based electronic data capture tools hosted at the University of Bristol (57).

A wide range of social, lifestyle, clinical and biological data have been collected on all family members repeatedly, including videos of parent-child interactions recorded using the headcams. Recruitment of mothers into the headcams study began on 7th July 2016, with 422 (90%) of mothers and their infants attending a 6-months assessment at the research clinic. 266 (63%) of mothers who attended the clinic were invited to record interactions with their infant using the headcams at home. 141 (53%) of these mothers consented to participate and 104 (74%) mothers provided video footage of mother-infant interactions. Initially, biological fathers and mothers' partners were invited to participate in the headcams study indirectly through an invitation to the mother when their child joined ALSPAC-G2. On 22nd July 2019, through additional funding from Wellcome Trust, a separate research clinic for fathers was set up (Focus on Fathers) inviting fathers directly to attend a range of assessments, including the headcams, when their G2 child was six months old. Overall, 194 fathers were invited to attend, with 83 (43%) fathers consenting to participate and 34 (18%) fathers providing video footage of father-infant interactions. For the purposes of this study, 25 mother-infant and 25 father-infant dyads were fully coded on various aspects of parental and infant vocalisations using the micro-behavioural observation coding system (58).

We captured video and audio footage of mother- and father-infant interactions using the headcams previously used for recording infant's eye view of their environment (59). The headcams are worn on headbands by both the parent (mother and father) and the infant, capturing two separate videos from the parent and infant perspective for each interaction. Headcams have previously been shown to be reliable for capturing mother and infant behaviours (52) and have been extensively used with fathers in the ALSPAC cohort. A questionnaire enquiring about fathers' experiences of using the headcams suggested that fathers perceive them to be user-friendly with no bearing on how they or their infant engage in the interaction. Separate headcam footage from both the parent and infant cameras were synchronised by the researchers for coding purposes. Headcam protocols were identical for both mothers and fathers. Parents were given fully-charged headcams and asked to use them at home during mealtime and play interactions. For the mothers, interactions analysed in this study were classed as “mealtime” (infant engages in eating = 24) and “stacking task” (mother and infant engage in a play task with a stacking toy; n = 1). For fathers, the interactions were classed as “mealtime” (n = 15); “stacking task” (n = 4), and “free play task” (father and infant engage in a play as they normally would; n = 6). Examples of each activity are presented in the MHINT micro-coding scheme (58). All videos were recorded at participants' home. Thus, it was possible for siblings/other caregivers/pets to be present during the interactions. Only three out of 25 children were from the same mother-father-child triad. Videos for 25 distinct mother-infant, and 25 distinct father-infant dyads (one father provided two interactions) were used in the present analyses.

First, we examined characteristics of our sample by parental and child characteristics (Table 2) and frequency and duration of parental and infant vocalisation behaviours (Table 3) using χ² and ANOVA tests. We ran Pearson correlations to examine the associations between maternal and paternal PND and infant vocalisations. Second, we conducted linear regressions to examine the associations between maternal and paternal PND (continuous exposures) and the frequency (mean rate per minute) and mean duration of parental vocalisation behaviours as continuous outcomes. Parental vocalisations that violated the assumptions of linear regression or were uncommon (e.g., criticism, intrusiveness, discouragement) were either dichotomised using a median split or divided into equal quintiles to derive ordinal variables. We then conducted ordinal logistic regressions and logistic regressions to examine the odds of being in a lower quintile or a category with a lower frequency or mean duration of each vocalisation behaviour as parental PND score increased. All models were first estimated unadjusted (exposure and outcome only), following incremental adjustment for amount of speech directed to infant, frequency/mean RPM/mean duration of infant vocalisation, and parental and child characteristics. Hypotheses-led analyses included models examining associations between maternal and paternal depression and frequency and duration of parental (1) all speech; (2) infant-directed speech; (3) attuned mind-mindedness; (4) positive emotional tone; (5) negative emotional tone; and (6) neutral emotional tone. The associations between maternal and paternal PND and the frequency and duration of all other parental vocalisation behaviours were examined without specific hypotheses. All analyses were conducted in Stata v.15 (Stata Corp., Texas, USA) (70) A sensitivity power analysis conducted using G*Power 3.1 (71) indicated that with a sample size of 25, α = 0.05, and 80% power, the minimum effect sizes this study was powered to detect were β = 0.51 and POR = 3.56. However, given that multiple tests were conducted, effect sizes at and above these thresholds could still be underpowered and findings were interpreted with caution.

In summary, the majority of children in our sample were first-born, of similar age, with a higher proportion of girls than boys. The average age of mothers and fathers was lower than that reported nationally in the UK [30.7 and 33.6 respectively; (72)] with mothers reporting higher educational attainment compared to fathers. The mean depression score (EPDS) for mothers (Mean = 6.61, SD = 5.37) was similar to that previously reported [Mean = 6.34, SD = 4.33; (73)], while the mean depression score for fathers in our sample (Mean = 8.17, SD = 5.11) was higher in comparison with previous research [Mean = 4.35, SD = 3.72; (73)] (Table 2).

Frequency, mean RPM, and mean duration of parental and infant vocalisation behaviours are displayed in Table 3. Overall, there were no differences in frequency and duration of infant vocalisation behaviours during father-infant, compared to mother-infant, interactions. Non-distress and any vocalisation (any verbal sound made by the infant, including distress, non-distress, screaming, bodily sounds, babbling and first words) were the most frequent behaviours displayed by the infants. Similarly, there were no differences between frequency and duration of maternal and paternal vocalisation behaviours when interacting with their infants. The most frequently demonstrated maternal and paternal vocalisations were speech, non-verbal sounds, silence and laughter, which also had the longest duration. With regard to modifiers of parental speech, the most frequent sentence structures were statements and questions, with both mothers and fathers acknowledging their infants during interactions. Fathers demonstrated higher frequency and mean RPM of commands and exclamations compared to mothers. Both mothers and fathers used positive and neutral tone, with somewhat higher mean duration of neutral tone displayed by the mothers compared to fathers. Negative and ironic/sarcastic tone was a rare feature of parental speech, although there was some indication that fathers used ironic/sarcastic tone more frequently than mothers. Both mothers and fathers engaged in adult- and infant-directed speech, although mean duration of adult-directed speech was higher, and mean duration of infant directed speech was lower, in fathers compared to mothers. Both mothers and fathers praised their infants during interactions, however, the frequency, mean RPM and duration of criticism directed toward the infant was higher in fathers than mothers. Similarly, frequency and mean duration of intrusive vocalisation behaviours was higher in fathers compared to mothers. Despite higher frequency and duration of criticism and intrusiveness, fathers engaged in attuned mind-mindedness more often, and for longer duration, compared to mothers. High frequency of infant-directed encouragement and low frequency of infant discouragement was displayed by both mothers and fathers. Frequency and mean RPM of verbal play was higher in fathers than mothers. Both mothers and fathers addressed their infants by their real name more often than the nickname, with some indications that fathers had higher mean RPM of using infant's real name compared to mothers. Both frequency and mean duration of teaching was higher in mothers compared to fathers. There was no evidence to suggest that paternal and maternal PND were correlated with the frequency of infant vocalisation behaviours, including distress, non-distress or any infant vocalisation (Table 4).

There was no evidence for associations between maternal PND and the majority of vocalisations in hypothesis-led analyses. There was evidence for a reduction in the frequency of maternal neutral tone as maternal PND increased in the unadjusted and adjusted models (β: −0.11, 95% CI: −0.20, −0.02, p = 0.016; Tables 5, 6). There was also evidence that increases in maternal PND were associated with increased odds of being in a quantile with higher duration of positive tone in models adjusting for amount of speech directed to the infant and duration of infant vocalisation (POR: 1.20, 95% CI: 1.02, 1.40, p = 0.026), and fully adjusted models (POR: 1.28, 95% CI: 1.04, 1.56, p = 0.017). There were fewer than five mothers displaying negative tone, precluding from examining associations with PND using inferential statistics.

Within hypothesis-free analyses of maternal vocalisations, there was only evidence for an association between maternal PND and duration of maternal encouragement. After adjusting for amount of speech directed to the infant, there was evidence that increased maternal PND was associated with increased odds of being in a quantile that encouraged their children for longer periods of time (POR: 0.08, 95% CI: 0.01, 0.15, p = 0.036). There was also weak evidence for this association when adjusting for amount of speech directed at the infant and mean duration of infant vocalisation (POR: 0.07, 95% CI: −0.003, 0.15, p = 0.06).

Similarly to mothers, there was no evidence of associations between paternal PND and the majority of paternal vocalisation behaviours in hypotheses-led analyses (Tables 5, 6). However, there was some evidence that paternal PND was associated with higher odds of being in a quantile with higher frequency of paternal positive tone in the unadjusted and fully adjusted ordered logistic regression models [Proportional Odds Ratio (POR): 1.28, 95% CI: 1.02, 1.61, p = 0.037; Table 6], although confidence intervals were wide. Paternal PND was also associated with higher odds of being in a quantile with higher mean duration of paternal speech (POR: 1.28, 95% CI: 1.01, 1.63, p = 0.045; Table 6) and infant-directed speech (POR: 1.27, 95% CI: 1.00, 1.61, p = 0.046; Table 6), however this was only apparent in fully adjusted ordered logistic regressions models accounting for maternal, paternal and child characteristics.

Hypotheses-free analyses of modifiers of paternal speech revealed no evidence for associations between paternal PND and the majority of paternal speech modifiers (Table 7). However, paternal PND was associated with higher odds of being in a quantile with higher frequency (POR: 1.52, 95% CI: 1.13, 2.05, p = 0.005; Table 7) and mean duration (POR: 1.63, 95% CI: 1.18, 2.25, p = 0.003; Table 7) of paternal laugh in the unadjusted and fully adjusted ordered logistic regression models. Paternal PND was also associated with higher odds of being in a quantile with higher mean duration of paternal question (POR: 1.79, 95% CI: 1.19, 2.70, p = 0.005; Table 7) and encouragement (POR: 1.25, 95% CI: 0.98, 1.60, p = 0.073; Table 7), however, this was only evident in fully adjusted ordered logistic regression models accounting for maternal, paternal and child characteristics.

There was no evidence to suggest that paternal PND was associated with any of the rare modifiers of paternal speech, including paternal discouragement and criticism of the infant, negative and ironic/sarcastic tone, intrusiveness, use of humour and teaching the infant (Table 8).

In line with previous research, we found that there were more similarities than differences between maternal and paternal vocalisation behaviours (45). Fathers and mothers both engage in equal frequency and duration of vocalisation behaviours when interacting with their infants, with speech, non-verbal sounds, silence and laughter being the most frequent vocalisations. There were also similarities in sentence structure, such as use of statements and questions, with both mothers and fathers acknowledging their infant during interactions. Both mothers and fathers used positive and neutral tone, although the mean duration of maternal neutral tone was higher than that for fathers. High frequency of encouragement of infant behaviours and low frequency of infant discouragement was observed in both mothers and fathers, with both addressing their infants by their real name more frequently than a nickname.

There were also some notable differences between maternal and paternal vocalisation behaviours. For instance, fathers demonstrated higher frequency and mean duration of commands and exclamation compared to mothers, while mothers engaged in more teaching compared to fathers. Existing research on content and function of maternal and paternal speech has documented a similar pattern, with fathers producing more direct (74) and prohibition commands than mothers (75). In line with previous research, fathers in our study engaged in more verbal play (both frequency and mean duration) compared to mothers (76). Adult- and infant-directed speech was a feature of both maternal and paternal vocalisation behaviours, though the mean duration of adult-directed speech was higher and that of infant-directed speech was lower in fathers compared to mothers. Existing research on paternal infant-directed speech to preverbal infants is rare. There is some evidence to suggest that, similarly to mothers, fathers make some prosodic modifications, but to a lesser extent. This is possibly due to men's lower average pitch compared to women's (45, 77).

Even though negative and ironic/sarcastic tone was a rare feature of parental speech, there was some evidence that this was more frequently displayed by fathers than mothers. This finding is consistent with earlier studies suggesting that fathers are more likely to tease their children compared to mothers (78). Similarly, frequency and duration of both criticism of the child and verbal intrusiveness was higher in fathers compared to mothers. Given the exploratory nature of our study, these findings should be interpreted with caution. However, these findings are in line with existing research suggesting that fathers may be less sensitive, more intrusive, directive and parent-centred than mothers during parent-infant interactions (79). It is possible that fathers engage in a more direct, stimulating, and challenging style of father-child interactions, so called “activation” parenting (80), which may seem more intrusive, but is important for promoting social and cognitive competencies in children (117). It has been previously argued that moderate levels of intrusiveness combined with positive and challenging stimulation in a supportive context that characterise fathers' interactional style allows children to explore new horizons while feeling safe and protected (82–84). It has also been argued that maternal and paternal sensitivity may be expressed differently through emotional warmth (mothers) and physical stimulation and playful interactions [fathers; (85)]. It has been argued that these two types of experiences chart the “rhythm of safety” vs. the “rhythm of exploration”, both of which are essential for healthy infant development (86). Future research that examines sequences of parental and child behaviours will enable us to examine behavioural manifestations of paternal intrusiveness in combination with other interaction behaviours (e.g., positive and/or negative affect) and, subsequently, their potentially differential effect on child development (87). This may lead to a reframing in how intrusive behaviour is defined and applied in the context of father-child interactions, and what it means for child development.

Despite higher frequency and duration of criticism and intrusiveness, fathers in our study engaged in mind-mindedness more often and for longer duration than mothers. Existing research on mind-minded speech in mothers and fathers is somewhat inconsistent, with some studies reporting no differences in the overall frequency of mind-minded comments between mothers and fathers (88), while other earlier studies noting more attentive utterances produced by fathers compared to mothers (89).

Based on existing literature, we hypothesised that the frequency and duration of maternal and paternal speech, as well as infant-directed and mind-minded speech and positive emotional tone will decrease, while the frequency and duration of negative and neutral emotional tone will increase in the context of parental depression. Our findings did not fully support these hypotheses. As hypothesised, there was evidence that as maternal PND increased, the frequency of maternal neutral tone decreased. We found no evidence for an association between paternal PND and use of a neutral tone. However, contrary to our original hypotheses, higher levels of maternal and paternal PND were associated with increased duration of parental positive tone, encouragement and laughter, all of which may be vocal proxies for more positive and sensitive interactions. Similarly, we found evidence to suggest that higher levels of paternal PND were also associated with increased duration of paternal speech, as well as infant-directed speech and questions, which may be suggestive of more infant-centred interactions.

Several explanations may be put forward to contextualise our findings. Existing research suggests that fathers with clinical levels of depression may engage in less positive parenting practices, and other enriching language activities with the child such as reading, singing songs and telling stories (90). However, effects of paternal PND on parenting, including vocalisation behaviours, may be more subtle in community samples with relatively mild levels of depression (91), and may not necessarily present themselves as overtly negative vocalisations (overall rare in our sample). Both neutral and positive tone were a predominant feature of maternal and paternal speech, whilst negative tone was rare with only 4% of fathers and 5% of mothers using a negative tone. It may be that there was more variability in neutral and positive tone vocalisations to detect effects associated with parental PND, with the opposite true for the negative parental tone. It is also possible that those parents who experience mild levels of PND have more emotional insight and empathy, and, thus, engage in more sensitive parenting, including vocalisation behaviours. It is possible that parents with increased PND try to compensate for their low mood by speaking in a positive tone, and their tone may not reflect genuine positive emotion. It is possible that more global qualitative coding such as the Emotional Availability Scales which capture an overall emotional tone would find differential results and this would be an interesting future direction to test this hypothesis. In addition, the focus of our analyses was the associations between parental PND and the frequency and duration of parental vocalisation behaviours, but sequences and patterns of behaviours may better capture patterns of expressed depressed mood.

Another contextual framework to explain our findings may be the changing nature of beliefs and attitudes surrounding contemporary parenting practices. Sociological literature has meticulously documented changes in parenting that occurred in the last forty years (92). The phenomenon of intensive or hyper-parenting (93), a highly demanding and child-centred approach to parenting, which includes heightened parental awareness of the adverse consequences of “bad parenting” ranging from poor educational to developmental outcomes (93, 94). Consequently, the process of raising a child through “concerted cultivation” (95) and attentive parenting may offer a way of mitigating the risks (94), including those associated with adverse parental mental health [for the critique and contested nature of such parenting practices see (96–99)]. Although the term “parenting” may be gender-neutral, the lens of intensive parenting has been predominantly focused on mothers as the primary agents responsible for shaping child outcomes (98, 100). However, the expectation that fathers should also be involved in parenting and childcare has also become prevailing in contemporary society (101, 102), with the demands of intensive parenting, albeit not to the same degree as mothers (101, 103), also extending to men. Intensive parenting is embedded in middle-class values (104), with “concerted cultivation” made possible through higher levels of financial and educational resources, with higher parental socio-economic status being consistently associated with more positive and consistent parenting styles and practices (105). Based on this evidence, it may be possible that both mothers and fathers in our study modified their vocalisation behaviours in line with their knowledge of the effects of mental health on parenting and the child, or there were more likely to engage in attentive and child-centred parenting in line with contemporary assumptions on what it means to be a “good” parent. Future longitudinal research with larger samples is needed to corroborate our findings regarding parental depression and vocalisation behaviours, particularly in the context of beliefs and expectations surrounding contemporary parenting practices.

One of the strengths of the study is the assessment of both maternal and paternal PND and vocalisation behaviours in early infancy. Most of the research to date has focused on maternal PND and its impact on parenting (2), with only a few studies addressing the impact of paternal PND on vocalisation behaviours during father-infant interactions. This approach continues to perpetuate a potentially problematic stance in developmental psychology, placing the mother-infant relationship at the cornerstone of human development. Similarly to mothers, features of father-infant vocalisation interactions may also constitute a transmission pathway by which paternal PND impacts the child. This argument is in line with the recently articulated stance to consider both maternal and paternal mental health from a family system perspective for both research and intervention purposes (106). Our comparisons of maternal and paternal vocalisation behaviours suggest that there are more similarities than differences between maternal and paternal vocal interactions with the infant, with comparative effects of parental PND on some aspects of vocalisation behaviours.

The use of observational rather than parent-reported measures to assess parental vocalisation behaviours is another strength of this study. The association between parental PND and the reporting of parenting has been consistently supported (107). In addition, there is some evidence to suggest that the headcams may be better at capturing vocalisation behaviours compared to “gold standard” observational methods due to the build-in microphones positioned closer to the participants' face (52). Importantly, the use of headcams is more likely to reduce participant reactivity and demand characteristics, enabling us to capture less socially desirable behaviours, including parental vocalisations (52). Although the advantage of reducing demand characteristics is not specific to the headcams, recording interactions in the familiar setting of a home context without a researcher present may facilitate capturing more variability in parental behaviours, compared to traditional observational methods. The use of the headcams may also reduce parental social anxiety and feelings of being judged, particularly in those parents who experience depression, compared to “gold standard” observational methods with a researcher filming the interactions, facilitating more natural positive responses to the child.

We captured a wide range of parental vocalisation behaviours, including a more detailed categorisation of each vocalisation withing a wider behavioural group using event-based micro-coding scheme (58). Arguably, the micro-coding systems may be better at highlighting complex patterns of dyadic interactions, capturing behaviours from both parents and infants as active participants in the interaction (108). It has also been argued that in comparison to global ratings, the systematic nature of micro-behavioural coding allows for the capture of more precise information on the nature of the observed behaviours (109). Indeed, our micro-coding behavioural scheme enabled us to capture multiple dimensions of parental and infant vocalisation with unprecedented degree of granularity regarding parental vocalisation behaviours (110, 111). Given the lack of evidence regarding the impact of maternal and, particularly, paternal depression on their vocalisation behaviours, such a degree of detail is particularly important.

The detailed nature of the micro-coding scheme has also enabled us to build a comprehensive comparison of maternal and paternal vocalisation behaviours across a range of dimensions, including their variation in intensity and frequency. Arguably, the use of headcams during mealtime and play interactions as they naturally take place at home, combined a multidimensional and versatile micro-coding behavioural scheme may be a better methodological paradigm to capture and assess the dynamic and transactional nature of parent-child interactions.

It should be noted, however, that our study is explorative in nature and findings should be interpreted with caution due to the fact that the study was not adequately powered to detect the observed effect sizes – the relatively small sample size and multiple testing with parental vocalisations as outcomes increased the possibility of committing a type 1 error. In addition, vocalisation behaviours made by fewer than five mothers and fathers had to be removed from the analyses, thus, associations between maternal and paternal PND and such behaviours could not be investigated. Further replication studies with a larger sample are needed to substantiate our preliminary findings, as well as provide more insights into the effects of parental PND on more rare aspects of parental vocalisations (e.g., maternal criticism of infant and intrusiveness). In addition, there were some discrepancies between the nature of the tasks completed between mothers and fathers, with mothers predominantly engaging in mealtime and stacking tasks, whilst a proportion of fathers have also engaged in a free play task. This could potentially elicit different type of parental vocalisation behaviours, with more goal-oriented interactions (e.g., mealtime and stacking task) eliciting vocalisations that are not necessarily generalisable across all parent-child interactions. Fathers engaging in free play task may also elicit parental vocalisations that are not directly comparable to less physically arousing and task-oriented interactions that mothers and infants accomplished. However, recent comparisons of studies between maternal and paternal vocalisation behaviours suggest that, despite some differences, both mothers and fathers modify their speech depending on the nature of the interaction (45).

Another limitation of the study relates to potential selection bias. The ALSPAC cohort is now a three-generational study, comprising “G0”: the cohort of original pregnant women, the biological father and other carers/partners, “G1”: the cohort of index children, and “G2”: the cohort of offspring of the index children, from which our study sample was drawn. The G0 mothers are overall from somewhat higher socio-economic background compared to the general population, whilst G1 participants who enrolled their children in G2 are more engaged with the ALSPAC study and more educated compared to those who did not participate in the study (55). By design, G1 participants fall within a restricted age range, with maternal age being further restricted by missing very young mothers who were not recruited, and the average age of mothers and fathers in this sample was lower than that reported nationally in the UK [30.7 and 33.6 respectively; (72)]. Both maternal age and education have been previously found to be important confounders in the association between parental depression, offspring outcomes and parenting (68, 63). It may be possible that infants of older and more educated parents are less likely to be exposed to specific manifestations of reduced parental insensitivity, including more negative vocalisation behaviours, associated with parental depression (112, 113). Fathers in our sample had higher mean depression scores than those previously reported, with men scoring on average 2 points above women. Anecdotally, it may be possible that fathers who experienced mental health difficulties were more likely to engage with the headcams study because of the insights it may bring into their parenting and potential impact on the child.

Associations among parental mental health, parenting and offspring development are complex and bidirectional (114). In line with transactional developmental models (115), children with more difficult temperament may influence maternal and paternal PND, as well as parental behavioural responses (116). We attempted to account for possible evocative effects by adjusting our analyses for the frequency and duration of infant vocalisation. However, addressing the possible bidirectionality (117) was outside the scope of the present study.

Pathways between maternal and paternal PND, parenting behaviours and offspring outcomes are complex and not fully elucidated. Our findings relate to one aspect of these complex relationships, notably the impact of parental PND on specific manifestations of vocalisation behaviours. Descriptively, we found more similarities than differences between maternal and paternal vocal interactions with their infants, with comparative effects of parental PND on some aspects of vocalisation behaviours, notably positive speech tone, encouragement and laughter. Our findings that higher levels of maternal and paternal PND were associated with increased duration of these behavioural proxies for more positive and sensitive interactions are tentative and implications of the findings are limited. Future replication efforts should focus on larger population-based samples that capture more variability in parental vocalisation behaviours, as well as contemporary beliefs and attitudes that define “good” parenting practices. These findings may indicate that the existing associations between PND and reductions in parental sensitivity require further detailed research.

Future research avenues should also focus on examining associations between specific aspects of parental vocalisation behaviours and more global measures of parental sensitivity to provide further insights into behavioural manifestations of warm and responsive parenting, particularly in the context of parental PND. Better understanding of specific behavioural manifestations of parenting and the overall quality of parent-child relationship and interactions may provide insights into the nature of difficulties that characterise early parent-child interactions, as well as key differences in maternal and paternal behaviours that may indicate depressed mood (61). In addition, the evidence-base regarding the effects of specific parental vocalisation behaviours, as well as patterns and sequences of such vocalisations, on offspring outcomes is lacking and should be strengthened through further investigations.

The coding scheme (58) applied in this study captures an unprecedented range of parental and infant behaviours (e.g., facial expressions, proximity) which should also be addressed in future investigations as potential markers of parental sensitivity which may be affected by parental depression. Traditionally, the main focus in parenting studies has been on mothers, with assessment of paternal parenting based on assessment of maternal parenting and mother-child relationships. Although such strategies may be useful to capture broader aspects of parenting, such as sensitivity and responsiveness, increasingly evidence suggests that the maternal template as a dominant methodology does not capture behaviours that may be unique to fathers, modelling parental sensitivity almost exclusively on maternal behaviours (80). The detailed nature of our coding scheme enables us to build a comprehensive picture of both maternal and paternal behaviours and to capture both differences and similarities in such behaviours across a range of parent-child interactions. It should be noted that even though families may be viewed as organized systems, each individual, including the infant, is an active, contributing member and part of the process that creates and maintains behavioural patterns (117, 118). Thus, the impact of infant temperament and behaviour on parental vocalization behaviours across a range of developmental stages and task scenarios in the context of parental mental health should also be studied.