Introduction
Although fathers in Western countries play an important role in child development (Lamb,
2010) and have increased their participation in child rearing over the past decades (Bakermans-Kranenburg et al.,
2019), they still spend less than half the amount of time on childcare compared to mothers (Huerta et al.,
2013). Moreover, fathers generally seem to show lower parenting sensitivity than mothers in the first years after the child’s birth (e.g., Hallers-Haalboom et al.,
2017). Fathers and children may thus benefit from interventions aimed at enhancing early paternal caregiving. Previous research showed that close physical contact between mothers and children induced by the use of a soft baby carrier or sling may positively affect maternal parenting and child attachment security (Anisfeld et al.,
1990). In the current randomized controlled trial (RCT), preregistered on
https://osf.io/qwe3a, we examined effects of the use of a baby carrier on fathers’ parenting behavior and hormonal functioning. Our research questions were: What are the effects of baby carrying on fathers’ interactive behavior with their baby, i.e., sensitivity and involvement, and hormonal functioning, i.e., basal oxytocin and cortisol levels and oxytocin and cortisol reactivity in response to interacting with the infant?
Over the past decades, there has been a growing awareness of fathers’ importance in child rearing, concurrent with increases in participation of fathers in childcare as well as in research focusing on fathering (Bakermans-Kranenburg et al.,
2019). Despite these developments, research on parenting still tends to focus mostly on mothers, possibly because mothers are often still seen as primary caregivers as they generally spend more time with children (Schoppe‐Sullivan & Fagan.
2020). In line with this focus on mothers, research on programs and interventions to stimulate paternal caregiving has been very limited. Nevertheless, such research is essential given the impact of paternal caregiving on child outcomes. More specifically, both paternal caregiving quality, i.e., paternal sensitivity, and quantity, i.e., paternal involvement, have been shown to be important for child outcomes (e.g., Lucassen et al.,
2011; Sarkadi et al.,
2008).
Paternal sensitivity is defined as fathers’ ability to perceive, adequately interpret, and appropriately and promptly respond to child signals (Ainsworth et al.,
1974). Children whose fathers are more sensitive show, for example, better cognitive functioning, better emotion regulation, less externalizing behaviors and more attachment security (Lucassen et al.,
2011; Rodrigues et al.,
2021).
Paternal involvement is a multi-faceted construct that encompasses the amount of time fathers directly engage with their child, i.e., time spent in one-on-one interaction with the child such as in play or physical care, and are available or accessible to their child (i.e., being present without necessarily directly interacting; Lamb et al.,
1985), but also the amount of time fathers spend thinking or communicating about or with their child (cognitive/affective involvement; Hawkins & Palkovitz,
1999). Research has shown that father involvement positively affects social, behavioral, psychological and cognitive child outcomes (Sarkadi et al.,
2008). Those effects were mostly examined and reported for paternal engagement, although some studies also indicated positive effects of paternal accessibility or of an overall involvement measure combining different aspects (i.e., being accessible, engaged and responsible) (Sarkadi et al.,
2008). Importantly, it has been shown that paternal sensitivity and involvement early in the child’s life is relevant for later child outcomes (Brown et al.,
2012).
Because of the relevance of early paternal caregiving for child development, it is important to examine ways in which parental caregiving may be improved. Several studies found positive effects of close physical contact, such as skin-to-skin contact, on infants (e.g., infant crying; Erlandsson et al.,
2007), and on parenting of both fathers (Chen et al.,
2017) and mothers (e.g., Bigelow et al.,
2010), even years later (Bigelow et al.,
2018). Parent-infant physical contact can also be facilitated by baby carrying. Although research on the use and the effects of the use of baby carriers in fathers is virtually non-existent (but see Riem et al.,
2021 for effects on fathers’ amygdala reactivity to infant crying in the current sample), studies in mothers have suggested that baby carrying may promote maternal caregiving and infant outcomes. One study in a sample of 49 mothers indicated that baby carrying may positively affect maternal behavior. Mothers who used a baby carrier showed higher responsiveness to infant signals at 3.5 months of infant age compared to mothers using a baby seat (Anisfeld et al.,
1990). Baby carrying mothers were not significantly more sensitive than mothers using a baby seat, but the moderate effect size for sensitivity in this study seems to support continued evaluation of effects of baby carrying on parental sensitivity. Moreover, infants of mothers who used a baby carrier were significantly more securely attached when they were 13 months old, compared to infants of mothers using a baby seat and the effect size for attachment was large (Anisfeld et al.,
1990). Another study in a very small sample (
N = 33) found similar results (Williams & Turner,
2020), but the fact that attachment was assessed at a very young age (7 months) and with an atypical procedure (the Still Face Paradigm) precludes drawing firm conclusions. Of note, the studies by Anisfeld et al. (
1990) and Williams and Turner (
2020) did not include a pre-test assessment and therefore it remained unclear whether using a soft baby carrier led to changes over time. Including pre-test assessments in an RCT is recommended, in particular when sample sizes are modest (Venter et al.,
2002). Other studies (some with pre-test assessments) reported positive effects of baby carrying on infant crying (Hunziker & Barr,
1986) and on breastfeeding duration (Pisacane et al.,
2012; Little et al.,
2021). In addition, mothers tend to be more responsive to infant vocalizations when they carry their baby than during face-to-face interaction (Little et al.,
2019). Taken together, these previous studies suggest that promoting physical contact between parent and infant can positively affect caregiving behavior. We expected that using a baby carrier would enhance both fathers’ sensitivity and their overall involvement (i.e., engagement, accessibility, and cognitive/affective involvement).
Baby carrying may also affect parent physiology. Two of the hormones that may be relevant are oxytocin and cortisol. Oxytocin is a neuropeptide that is produced in the hypothalamus and is mostly known for its involvement in labor and lactation, although oxytocin also has anxiolytic effects and has been related to a variety of (social) behaviors (Ellis et al.,
2021). Cortisol is a steroid hormone secreted by the hypothalamic-pituitary-adrenal axis and is well known for its association with stress and stress regulation (Saxbe,
2008). Importantly, both oxytocin and cortisol have been associated with caregiving, such that oxytocin has been positively and cortisol negatively related to positive caregiving behavior (e.g., Bos et al.,
2018; Naber et al.,
2010). Increased oxytocin and decreased cortisol may therefore be relevant for caregiving behaviors and, in turn, for child outcomes. What is more, oxytocin and cortisol seem to be affected by physical contact (Field,
2010). Skin-to-skin contact of parents with their pre-term infants has been related to increases in oxytocin levels and decreases in cortisol levels measured with single saliva samples in both fathers and mothers (Bigelow et al.,
2012; Cong et al.,
2015; Vittner et al.,
2019). Additionally, fathers who engaged in stimulatory touch with their infants showed higher levels of baseline plasma and salivary oxytocin (measured with single samples) and increases in salivary oxytocin levels (Feldman et al.,
2010). Moreover, paternal salivary cortisol decreased when fathers held their infant (Kuo et al.,
2018). Frequent use of a baby carrier may affect several aspects of fathers’ hormonal functioning. First, enhanced physical contact between parent and infant may increase secretion of oxytocin and decrease secretion of cortisol, resulting in higher basal oxytocin levels and lower basal cortisol levels. Second, fathers using a soft baby carrier may become more attuned to their infants and therefore show higher increases in oxytocin levels and higher decreases in cortisol levels following interaction with their infant.
Fathers in the control condition of the current RCT used a baby seat. We chose the baby seat as the control condition because this was also the control condition in the Anisfeld et al. (
1990) study on the effects of using a baby carrier. Using a baby seat may stimulate fathers’ face-to-face interaction with their infant, but we do not expect any hormonal effects of using a bay seat because of the absence of close physical contact. Moreover, because in mothers it was found that they were more responsive to infant vocalizations when they carried their baby than during face-to-face interaction (Little et al.,
2019), we expected few (if any) effects of baby seat use on fathers’ sensitivity.
In this RCT, we examined the effects of baby carrying on fathers’ parenting behavior and hormonal functioning. Our primary hypothesis was (1) fathers in the baby carrier condition will show higher increases in sensitivity from pre-to post-intervention compared to fathers in the baby seat condition. Our secondary hypotheses were (2) fathers in the baby carrier condition will show higher increases in involvement from pre-to post-intervention compared to fathers in the baby seat condition; (3) fathers in the baby carrier condition will show increases in basal oxytocin levels and decreases in basal cortisol levels from pre- to post-test compared to fathers in the baby seat condition; (4) fathers in the baby carrier condition will show increased hormonal reactivity (i.e., higher increases in oxytocin levels and higher decreases in cortisol levels) in response to interacting with their infant from pre- to post-test compared to fathers in the baby seat condition. We additionally performed some exploratory analyses that were not preregistered. Specifically, we explored differences between the baby carrier and baby seat group in changes of fathers’ endorsement of parenting principles relating to regularity and routines (structure) and to infant cues and close physical contact (attunement). Finally, potential moderators of intervention effects on sensitivity and involvement, such as infant sex and reported tool-use time, were explored.
Discussion
This RCT, preregistered on
https://osf.io/qwe3a, was the first to examine the effects of a soft baby carrier intervention on fathers’ parenting behavior and hormonal functioning. The results showed that the intervention did not affect fathers’ sensitive parenting, although carrying time was moderately (but non-significantly) associated with fathers’ post-test sensitivity in the intervention group. The baby carrier intervention also did not promote fathers’ involvement. Involvement operationalized as hours spent with the infant decreased over time for fathers in the carrier condition compared to fathers in the control condition. The baby carrier intervention had no effect on fathers’ basal oxytocin or cortisol levels, nor did it affect fathers’ oxytocin and cortisol reactivity to interacting with their infant. Additionally, there were no effects on fathers’ endorsement of principles relating to structure and attunement. Exploratory moderation analyses revealed no moderators of the intervention effects on sensitivity and involvement.
No significant effect of baby carrying on paternal sensitivity was found in the current study. Using an inferiority test (Lakens et al.,
2018), we compared this study’s effect to a previous study in mothers showing a medium, albeit non-significant positive effect on sensitivity (Anisfeld et al.,
1990) and found that the current effect was statistically smaller than the previously reported effect in mothers,
t(68) = −3.17,
p < 0.01. Baby carrying may affect mothers differently than fathers, as sex-specific differences have been reported in parenting and its neurohormonal correlates (Rajhans et al.,
2019). Nevertheless, such differences seem attenuated when fathers are more attuned to and involved with their infants (Abraham et al.,
2014), which is exactly what the current baby carrier intervention aimed to do. Other studies did find positive effects of skin-to-skin contact on fathers’ feelings and behavior toward their baby (Chen et al.,
2017; Varela et al.,
2014), but it should be noted that the baby carrier did not promote skin-to-skin contact. Additional differences between the current study and the Anisfeld et al. (
1990) study may explain the diverging findings. Specifically, the Anisfeld et al. (
1990) sample consisted of mothers with a low socioeconomic status, whereas fathers in the current study were overall highly educated. Parents from lower socioeconomic backgrounds have been reported to show lower parenting sensitivity (Pelchat et al.,
2003) and examining effects of baby carrying in fathers with a lower socioeconomic background may be an interesting target for future research. Importantly, in the study by Anisfeld et al. (
1990) no pre-test was included and therefore it remained unclear whether using a soft baby carrier led to an increase in maternal sensitivity over time.
The baby carrier intervention did not promote fathers’ involvement with their infants. No effect was found on involvement assessed in real-time using a smartphone application. Unexpectedly, we found that fathers in the baby seat condition significantly increased in self-reported hours spent with their infant from pre- to post-test, whereas fathers in the baby carrier condition showed a non-significant decrease over time. This suggests that using a baby seat may positively influence how much time fathers spend with their child. Speculatively, enhanced face-to-face or playful interaction between father and infant through the use of a baby seat may stimulate father–child bonding and contribute to an increase in fathers’ time spent in caretaking activities (Premberg et al.,
2008). Also note that we had no information on whether the child was awake or asleep during tool use, and if children tended to fall asleep during carrying this would not promote further interaction between father and child. Future research should examine whether the increase in involvement in the seat condition and trend toward decrease in involvement in the carrier condition are maintained over a longer-term period.
Fathers’ basal oxytocin and cortisol levels were not affected by the baby carrier intervention, nor was their oxytocin and cortisol reactivity to interacting with their infant. These findings are not in line with previous studies suggesting that touch and skin-to-skin contact are related to increased oxytocin levels and decreased cortisol levels (e.g., Cong et al.,
2015; Feldman et al.,
2010; Field,
2010). Contrary to these previous studies, here we did not assess immediate effects of physical contact, but rather longer-term effects on hormonal basal levels and reactivity. Possibly, physical contact enhances oxytocin and decreases cortisol momentarily, but not on the longer term. In line with this suggestion, fathers’ cortisol levels were found to decrease during skin-to-skin contact with their infant, but increased again shortly after the skin-to-skin period ended (Cong et al.,
2015). Moreover, we note again that in our study we did not assess effects of skin-to-skin contact but rather of physical contact that was not directly skin-to-skin. Although both types of contact are physical, they are not the same, and hormonal levels may be more strongly affected by direct skin-to-skin contact. Interestingly, we found a decrease in oxytocin over time independent of intervention condition, suggesting that fathers’ oxytocin levels drop from 2–4 to 3–6 months postnatally. It is still largely unknown how fathers’ oxytocin levels may change throughout the postnatal phase. A previous study reported a rise in plasma oxytocin levels in first-time fathers from the first postpartum weeks to 6 months postpartum (Gordon et al.,
2010). This points to mixed findings and possibly large interindividual differences in hormonal changes exist. Future research may focus on examining how fathers’ hormonal levels vary during this period and on potential moderators of such changes.
Limitations and Future Directions
This study’s findings should be considered within the context of some limitations. First, fathers did not all adhere to the instruction to use the carrier for at least 6 h per week, spread over a minimum of 4 days, for 3 weeks. The average recorded time of use across the intervention period was less than 12 h, where it should have been 18 h if the fathers had followed the instructions. Intervention effects might have been stronger if program adherence had been higher. We did find moderate correlations within the carrier group of recorded carrying time with sensitivity, involvement, and cortisol reactivity at post-test, suggesting that fathers who used the carrier more intensively were more sensitive, more involved, and their cortisol levels decreased more during interacting with their infant. These correlations should be interpreted cautiously because they did not account for pre-test differences between fathers and they might be explained by other factors relating to both carrying time and outcome variables. The exploratory moderation analyses indicated that time using the tool did not significantly moderate the effects of the intervention, but this may also be due to compromised power for the moderation analyses. Future studies may consider adjusting the intervention schedule by for instance increasing the number of intervention weeks. Extending the intervention period will allow us to assess whether longer use of the baby carrier (and/or use with somewhat older infants) has more pronounced effects. As an alternative or complementary approach, parents may be contacted weekly to review their baby carrier use, so that use of the baby carrier can be reinforced and constructive feedback can be provided when they experience problems in (sufficient) use of the baby carrier. For many interventions, higher compliance predicts better outcomes (Berkel et al.,
2018; Clarke et al.,
2015).
Second, we had no information on baby carrier use of fathers in the control condition during the intervention period. We know that at the time of inclusion none of the participants used a carrier for over 5 h per week. Importantly, we asked fathers in both conditions approximately 4 months after the end of the intervention how much they had used an infant carrier on average per week over the past 4 months. Fathers in the carrier condition reported using a carrier more often than fathers in the control condition, with a medium effect size (Cohen’s d = 0.47). This suggests that fathers assigned to the carrier intervention used the infant carrier more often than fathers in the control condition in the months after the intervention and it is likely that this difference was at least similar during the intervention period. However, it remains important for future studies to rule out cross-contamination during the intervention period.
Third, we only assessed short-term effects of the intervention and therefore cannot speak to potential effects of baby carrying on the longer term. Research on the effects of the baby carrier intervention suggests that spending time in physical contact with the infant may promote fathers’ attention to infant signals (i.e., increases fathers’ amygdala reactivity to infant crying; Riem et al.,
2021). Possibly, this may stimulate sensitivity in the longer run, but this remains to be tested in future research.
Fourth, for the measurement of basal cortisol levels, it might have been optimal to increase sampling intensity, i.e., collect more than two samples per day, as this would have allowed us to account for cortisol’s diurnal rhythm which is not linear. However, given out-of-the-home work of most of our participants, this would have led to many missing data, and we therefore preferred collecting saliva four times across 2 days. Nevertheless, as sampling intensity may affect the accuracy of cortisol estimation, future studies should consider using a higher number of samplings per day (Hoyt et al.,
2016).
Fifth, we focused on fathers in the current study and not on both parents. Although less is known about paternal caregiving, it would be beneficial for future studies to take multiple caregivers (e.g., fathers and mothers) into account. Also, the fathers in this study were in heterosexual relationships, which may affect their parenting roles. Finally, we did not assess effects of the carrier intervention on infant outcomes, such as attachment security. Examining effects of fathers’ baby carrying on infants seems an interesting avenue for future research as previous studies on infant carrying in mothers and skin-to-skin contact in fathers reported positive effects on child outcomes (Anisfeld et al.,
1990; Shorey et al.,
2016).
Implications
Our findings have implications for those who want to promote paternal sensitive caregiving. In all, we did not find that baby carrier affected fathers’ caregiving behavior. It is important to examine whether interventions such as a baby carrier intervention can have positive effects on parental sensitivity, as this type of intervention requires less involvement of interveners and is easier and cheaper to implement than interventions that specifically target sensitive caregiving behavior (e.g., Video-feedback Intervention to promote Positive Parenting and Sensitive Discipline (VIPP-SD; Juffer et al.,
2008; see Buisman et al.,
2022 for effects of prenatal video-feedback using ultrasound (VIPP-PRE) on fathers’ sensitivity). When a baby carrier would be as effective as personalized feedback to promote sensitive parenting behavior, the lower costs of a baby carrier intervention may mean that it would be better in terms of cost-effectiveness. However, the current findings provide no indication that this less extensive type of intervention is effective at enhancing paternal sensitivity (at least not with the current carrying duration). Intervention programs may need to target parenting behavior more directly to positively affect it.
A theoretical issue is whether effects of a baby carrier intervention could be different for men and women, because of differential hormonal processes underlying parenting behavior. In support of this idea, increasing oxytocin levels were found to be related to different parenting behaviors in fathers and mothers (Feldman et al.,
2010). Nevertheless, as mentioned above, other explanations for the different outcomes for fathers in this study compared to Anisfeld et al. (
1990) results for mothers cannot be excluded. Additionally, it is unclear how much the mothers in that study used the infant carrier. If mothers in that study used the infant carrier more often and with longer duration than fathers in the current study, it may be beneficial in future research to more closely follow-up or more actively stimulate fathers to use the baby carrier more frequently during the intervention.
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