*3.2. Combined Effect of Breastfeeding and Active Bonding*

ANOVA results showed significant group differences on mean behavior scores for total internalizing (*F*(3,1264) = 5.21; *P* = 0.001), anxious/depression (*F*(3,1263) = 2.779; *P* = 0.04), somatic complains (*F*(3,1264) = 3.20; *P* = 0.023) and withdrawn (*F*(3,1264) = 6.75; *P* < 0.001). Emotionally reactive showed borderline significance (*F*(3,1261) = 2.38; *P* = 0.068). Mean (SD) scores, effect sizes (omega squared *w* 2 ) and *P* values from ANOVA for the four comparison groups are given in Table 2. Results revealed identical trend across all dependent variables, with group 1 displaying the lowest scores, followed by group 3. Group 4 had the highest scores.

## *3.3. Potential Confounds*

Demographic and social variables are known to influence breastfeeding and children's behavior. Our study showed that boys were more likely be fed by formula or a mixed method rather than pure breast milk (Ȥ 2 = 2.17; *P* = 0.031). Social adversity scores were significantly lower for those whom actively bonded with their baby while feeding (*t* = 5.18; *P* < 0.001), indicating they had a better socioeconomic background and health status (Table 1). Consequently, it is possible that social adversity and gender could account for the main effects of breastfeeding and bonding. We also tested the correlations between potential confounders and dependent variables. In our sample, Spearman correlation indicates social adversity was positively correlated to each syndrome and total internalizing problems (*P* < 0.001 for all), although no significant correlation was detected between gender and dependent variables. As gender and social adversity were correlated to either the breastfeeding and bonding types or the internalizing problems, we entered these two constructs in to a series of general linear models. The main effects remained significant for Anxious/Depressed, Somatic Complaints, Withdrawn and total internalizing (Table 3) after controlling for adversity (*F*(1,1252) *P* IRUDOODQGJHQGHU*F*(1,1252) 1.56; *P* 0.212 for all).

#### *3.4. Effect Moderator*

There was no interaction between breastfeeding and bonding grouping and gender (*F*(1,1252) 2.1; *P* 0.097 for all), indicating that this measure did not moderate the effects of breastfeeding and bonding.

#### *3.5. Dose Response Relationship*

Univariate ANOVAs (with three groups: 0–7 months, 7–10 months and 10 months duration levers) were conducted on each syndrome and total internalizing scores. Results showed significant main effect for breastfeeding duration on anxious/depression (*F*(2,1170) = 3.28; *P* = 0.038), somatic complains (*F*(2,1171) = 3.25; *P* = 0.039) and total internalizing (*F*(2,1171) = 2.99; *P* = 0.051). No significant dose-response effect was detected for emotionally reactive (*F*(2,1170) = 1.21; *P* = 0.298) and withdrawn (*F*(2,1171) = 0.910; *P* = 0.403). The mean scores for each syndrome and total internalizing problems were plotted against breastfeeding duration levels in Figure 1a,b.

### **4. Discussion**

Three key findings emerged from this study. First, compared to children whose mothers breastfed them, children who were not breastfed showed an increased number of internalizing behavioral problems, particularly anxious/depressed and somatic symptoms. Second, the group of children whose mothers both breastfed and actively interacted with their infants had the least likelihood of displaying internalizing problems. Children who were not breastfed but whose mothers still engaged in active interactions displayed the next-lowest risk, while being neither breastfed nor exposed to active bonding had the smallest effect on internalizing behaviors. Finally, a duration effect (dosage effect) appeared such that breastfeeding for 10 months or longer had the strongest impact on reducing anxious/depressed and somatic symptoms in children.

Breastfeeding confers a strong biological benefit to infants and their development [23]. From a nutritive standpoint, breast milk contains docosahexaenoic acid (DHA) omega-3 fats, the consumption of which, along with eicosapentaenoic acid (EPA) fats, may reduce the risk for affective disorders, including major depression and bipolar disorders, particularly among women [24,25]. However, the overall literature on DHA and depression remains mixed [26,27]. What is known, however, is that DHA plays a vital role in neural development, neurotransmitter transmission, and genetic expression, making it highly relevant to child neurodevelopment as well as developmental disorders, such as attention-deficit/hyperactivity disorder and motor deficits [28].


Note: significant results were highlighted in bold.


**Table 3.** General linear model statistics of breastfeeding type and bonding against internalizing problems, controlling for gender and social adversity.

Note: significant results (*P*<0.05) were highlighted in bold.

**Figure 1.** Dose-response Relationship between duration of breastfeeding and internalizing behavior. (**a**) Breastfeeding duration and internalizing syndromes; (**b**) Breastfeeding duration and total internalizing problems.

(**b**)

It may be that the biological benefit offered to breastfed infants plays a role in healthy cognitive maturation which in turn lowers their risk for psychopathology [29]. Feldman and Eidelman report that breastfeeding is associated with improved motor and social skills [5], but other authors have not found an impact on emotional regulation and behavioral disruption, indicating the need for further research on breastfeeding and child psychodevelopment [30]. Interestingly, a recent study examined effects of breastfeeding on mental health outcomes among children at age 14 years and found that breastfeeding at age six months was associated with a lower rate of child psychopathology, including social and attention difficulties and aggression [31]. However, more longitudinal data is needed to better understand the potential long-term benefits of breastfeeding to child mental health. Whether the nutritional, physiological and cognitive benefits from breastfeeding directly enhance mental health via a biological route in our brain may still require further exploration. However, speculatively, the reported nutritional, physiological and cognitive benefits can confer a lot of advantages to a child to negotiate with the challenges of growing up. For example, a healthier, energetic physical body and a faster cognitive growth can help a child to cope with the arduous demands of modern-day schooling, particularly in mainland China, the tradition of which has long emphasized education as the avenue for upward socio-economic migration. A child who excels at school will also be well liked and accepted by parents, relatives, teachers and peers. School success and social popularity are both known key precursors to mental health [32]. Thus, it is likely that there may be both a direct biological route and an indirect psychosocial route from breastfeeding leading to positive mental health or fewer internalizing problems. These dual routes should both be further examined in future study.

Breastfeeding also provides a biological benefit to the mother by reducing blood pressure and pain [33]. Furthermore, the release of hormones oxytocin and prolactin not only confer analgesic and relaxation benefits, but they also appear to play a key role in mother-infant bonding [11,33], which has been shown to reduce emotional and behavior problems in children [6,7].

The attachment aspect of breastfeeding underscores the need to consider its potential mental health benefits. Psychologically, breastfeeding may enhance the mother-infant bonding process via active talking, eye contact, and skin-to-skin touch. This may help mothers form stronger attachments to offspring and improve maternal sensitivity [33], reduce postpartum depression [34], and ward off other negative mood states like maternal stress [35]. This may indirectly benefit a child's mental health, as the literature detailing the impact of maternal depression on increasing the risk of future child and adolescent psychopathology is compelling [36,37].

Infants may similarly derive a mental health benefit from being breastfed, including development of more secure attachments and reduced negative temperament [38]. Several authors have documented analgesic properties of breast milk, along with reductions in salivary cortisol, due to milk odor and skin-to-skin contact [39,40]; these are hypothesized to help alleviate child distress and strengthen bonding.

Taken together, these findings underlie a biopsychological aspect of breastfeeding wherein the physiological benefits of breastfeeding (e.g., pain reduction, stress reduction, healthy cognitive development) coupled with improved pair bonding and mother-infant attachments may provide protective effects against the formation of child internalizing behaviors The biopsychosocial interaction may also provide indirect benefits that operate through mediating or moderating variables [41]. For example, secure parent-child attachment may improve child sleep quality [42], and reduced sleep problems in children has been linked to better emotional and behavioral functioning [43]. In addition, breastfeeding is ultimately a holistic process and there are several aspects that facilitate the process, including how the mother responds to the infant, the physical and social environment around the mother and baby, and the level nutrients in the breast milk. These factors, such as the genetic and environmental influences of nutrient intake (e.g., breast milk) should be considered [44]. Consideration of such in larger samples will require further study before definitive conclusions can be drawn about intervening variables on breastfeeding and internalizing conditions.

In this study, breastfeeding for a longer duration (at least 10 months) had the greatest effect on reducing internalizing symptoms. This is consistent with other authors who report that longer duration of breastfeeding was associated with greater protection against child mental health problems at age five years [45]. Another recent longitudinal cohort study from Oddy and colleagues followed breastfed infants to 14 years of age and found that breastfeeding for six months or less independently predicted greater externalizing and internalizing problems in childhood and adolescence, compared to infants who were breastfed for 6 months duration or longer [14].

Some important limitations on the present study's findings exist. First, the use of retrospective data may involve recall bias. However, in the current literature, it is not rare for studies examining breastfeeding practices to use maternal recall data after much longer periods. For example, a study by Promislow looked at maternal recall of breastfeeding duration of elderly US women from 34 to 50 years ago [46]. Nevertheless, future prospective designs should be considered. As previously mentioned, we also did not include holistic measures in our assessment of breastfeeding as we do not have data available due to the retrospective nature of this study. Instead, we included confounding factors, such as demographic and social background, in our analysis of breastfeeding. Another limitation of the present study is that it does not take into consideration the exact duration of exclusive breastfeeding, which is particularly important given the fact that breastfeeding practices have decreased in recent years, especially among urban and well-educated mothers [47]. Future studies should test if there is a duration-dependent relationship between breastfeeding and internalizing behavior in children. Additionally, as active bonding was one of the key predictors, future studies should employ validated, empirical-based measures on this construct. However, despite the use of such "crude" measures, they are able to produce consistent results, indicating evidently the benefits of breastfeeding on the children's mental health. Furthermore, future studies should stratify by region, given that breastfeeding practices differ by location.
