Sex differences in left-side infant holding: Results from “family album” photographs

Functional lateralization in social-emotional processing: The influence of sexual orientation and gender identity on cradling preferences

The left-cradling bias (i.e., the motor asymmetry for cradling infants on the left side) has often been associated to the right-hemispheric social-emotional specialization, and it has often been reported to be stronger in females than in males. In this study we explored the effects of sexual orientation and gender identity on this lateral bias by means of a web-based investigation in a sample of adults (485 biological females and 196 biological males) recruited through LGBTQIA+ networks and general university forums. We exploited a cradling imagery task to assess participants' cradling-side preference, and standardized questionnaires to assess participants' homosexuality (Klein Sexual Orientation Grid) and gender nonconformity (Gender Identity/Gender Dysphoria Questionnaire for Adults and Adolescents). Results confirmed the expected left-cradling bias across all sexual orientation groups except for heterosexual males. Importantly, higher homosexuality scores were associated with higher proportions of left cradling in males. These results suggest that sexual orientation can influence cradling preference in males, indicating a complex interaction between biological and psychological factors in the laterality of social-emotional processing. Finally, the left-cradling bias seems to confirm its role as a behavioral proxy of social-emotional functional lateralization in humans.

Artistic turns: laterality in paintings of kisses and embraces

Many lateral biases exist in human behavior, often implicit and not deliberated. Romantic kissing and embracing received experimental attention in the last three decades. We investigated laterality in paintings depicting these social interactions using two methodologies to assess whether painters depicted such biases and whether these biases could be due to observers' aesthetic preferences or painters' ability in portraying naturalistic interactions. In Study 1, we inspected about 190,000 artworks available online to classify leftward and rightward biases in romantic kisses and embraces. The comparison of 103 paintings depicting clearly lateralized interactions revealed a significant rightward bias in romantic kissing (66%) and a trend toward a leftward bias (62%) for embraces, aligning with naturalistic studies of human interactions. In Study 2, 128 participants expressed their aesthetic preference between the paintings selected in Study 1 and their vertically mirrored versions. A preference for the original paintings over their mirrored versions emerged, especially when presented in the upper portion of the screen, but no significant preference for the typical lateralization of kissing and embracing was found. These findings suggest that artists' alignment with naturalistic biases could be due to familiarity and exposure to asymmetric interactions rather than observers' aesthetic preferences.

Maternal cradling bias: A marker of the nature of the mother-infant relationship

The tendency of mothers to cradle their infant on their left side was first demonstrated by Salk back in 1960, but has been the subject of renewed interest in recent years. In this paper, I examine the different aspects of this tendency, making a short detour to explore this behavioral bias in other mammals and in nonhuman primates. The left cradling bias has been investigated not only in mothers, but also in fathers, children and students, as well as in different cultures. Moreover, the origins and causes of this behavior have been explored by examining its links to hemispheric specialization and to other motor asymmetries in mothers and infants. In recent years, researchers have also explored divergent patterns of cradling in groups of mothers exhibiting stress, anxiety, or depressive symptoms during pregnancy and after delivery, as well as in children with atypical development (e.g., autism spectrum disorder). The findings reported in the past 20 years open up promising avenues for anticipating potential difficulties in the relationship between mother and child, for given that a left cradling bias has been found in about 75% of the samples tested, cradling patterns could constitute natural behavioral markers of the socio-emotional harmony between cradler and cradled.

Human Lateralization, Maternal Effects and Neurodevelopmental Disorders

In humans, behavioral laterality and hemispheric asymmetries are part of a complex biobehavioral system in which genetic factors have been repeatedly proposed as developmental determinants of both phenomena. However, no model solely based on genetic factors has proven conclusive, pushing towards the inclusion of environmental and epigenetic factors into the system. Moreover, it should be pointed out that epigenetic modulation might also account for why certain genes are expressed differently in parents and offspring. Here, we suggest the existence of a sensitive period in early postnatal development, during which the exposure to postural and motor lateral biases, expressed in interactive sensorimotor coordination with the caregiver, canalizes hemispheric lateralization in the “typical” direction. Despite newborns and infants showing their own inherent asymmetries, the canalizing effect of the interactive context owes most to adult caregivers (usually the mother), whose infant-directed lateralized behavior might have been specifically selected for as a population-level trait, functional to confer fitness to offspring. In particular, the case of the left-cradling bias (LCB; i.e., the population-level predisposition of mothers to hold their infants on the left side) represents an instance of behavioral trait exhibiting heritability along the maternal line, although no genetic investigation has been carried out so far. Recent evidence, moreover, seems to suggest that the reduction of this asymmetry is related to several unfavorable conditions, including neurodevelopmental disorders. Future studies are warranted to understand whether and how genetic and epigenetic factors affect the lateralization of early mother-infant interaction and the proneness of the offspring to neurodevelopmental disorders.

Received Cradling Bias During the First Year of Life: A Retrospective Study on Children With Typical and Atypical Development

A population-level left cradling bias exists whereby 60-90% of mothers hold their infants on the left side. This left biased positioning appears to be mutually beneficial to both the mother and the baby's brain organization for processing of socio-emotional stimuli. Previous research connected cradling asymmetries and Autism Spectrum Disorders (ASD), entailing impairment in socio-communicative relationships and characterized by an early hypo-lateralization of brain functions. In this explorative study, we aimed to provide a contribution to the retrospective investigations by looking for early behavioral markers of neurodevelopmental disorders such as ASD. We hypothesized that an atypical trajectory in maternal cradling might be one of the possible signs of an interference in mother-infant socio-emotional communication, and thus of potential neurodevelopmental dysfunctions. To this aim, we examined photos depicting mother-child early cradling interactions by consulting family albums of 27 children later diagnosed with ASD and 63 typically developing children. As regards the first half of the first year of life, no differences were shown between maternal cradling-side preferences in typical and ASD groups, both exhibiting the left-cradling bias in the 0-3 months period, but not in the 3-6 months period. However, our results show dissimilar patterns of cradling preferences during the second half of the first year of life. In particular, the absence of left-cradling shown in typical mothers was not observed in ASD mothers, who exhibited a significant left-cradling bias in the 6-12 months age group. This difference might reflect the fact that mother-infant relationship involving children later diagnosed with ASD might remain "basic" because mothers experience a lack of social activity in such children. Alternatively, it may reflect the overstimulation in which mothers try to engage infants in response to their lack of responsiveness and social initiative. However, further investigations are needed both to distinguish between these two possibilities and to define the role of early typical and reversed cradling experiences on neurodevelopment.

The association between received maternal cradling and neurodevelopment: Is left better?

The vast majority of women (60-90%) hold infants on their left side. Such a population-level lateral bias has been shown to improve the processing of socio-emotional stimuli in both the woman and the baby. Recently, some studies related cradling lateralization and Autism Spectrum Disorders (which entail socio-communicative deficits and a reduction of the typical lateralization of brain functions), raising the possibility that the asymmetrical cradling behavior experienced during infanthood might be related to the infant's neurodevelopment. Any progress made towards earlier diagnoses could significantly increase the chance of successful intervention for these patients. We here suggest that a wide retrospective investigation on family photo albums and home movies of children diagnosed with Autism might provide useful data about early behavioral signs of this condition. We hypothesize that an atypical trajectory in maternal cradling might be one of the early signs of interference in dyadic socio-emotional communication, and thus of potential neurodevelopmental dysfunctions.

The left-cradling bias and its relationship with empathy and depression

Women usually cradle their infants to the left of their body midline. Research showed that the left cradling could be altered by affective symptoms in mothers, so that right cradling might be associated with a reduced ability to become emotionally involved with the infant. In this study, we assessed cradling-side bias (using family photo inspection and an imagination task), as well as depression and empathy, in 50 healthy mothers of 0-3 years old children. The main finding was that the strength of the left-cradling bias was negatively related with participants' depression scores and slightly positively related with their empathy scores. Our results thus provide further evidence that cradling-side preferences can represent an evolutionary proxy of mother's affective state, influencing the early development of the infant social brain and behaviour.

The left cradling bias: An evolutionary facilitator of social cognition?

A robust left side cradling bias (LCB) in humans is argued to reflect an evolutionarily old left visual field bias and right hemisphere dominance for processing social stimuli. A left visual field bias for face processing, invoked via the LCB, is known to reflect a human population-level right cerebral hemisphere specialization for processing social stimuli. We explored the relationship between cradling side biases, hand dominance and socio-communicative abilities. Four and five year old typically-developing children (N = 98) participated in a battery of manual motor tasks interspersed by cradling trials comprising a(n): infant human doll, infant primate doll, proto-face pillow and no-face pillow. Mean social and communication ability scores were obtained via a survey completed by each child's key teacher. We found a population-level LCB for holding an infant human doll that was not influenced by hand dominance, sex, age or experience of having a younger sibling. Children demonstrating a LCB, did however, obtain a significantly higher mean social ability score compared with their right side cradling counterparts. Like the infant human doll, the proto-face pillow's schematic face symbol was sufficient to elicit a population-level LCB. By contrast, the infant primate doll elicited a population-level right side cradling bias, influenced by both hand dominance and sex. The findings suggest that the LCB is present and visible early in development and is likely therefore, to represent evolutionarily old domain-specific organization and function of the right cerebral hemisphere. Additionally, results suggest that a LCB requires minimal triggering but can be reversed in some situations, possibly in response to species-type or levels of novelty or stress as perceived by the viewer. Patterns of behavioral biases within the context of social stimuli and their associations with cognitive ability are important for understanding how socio-communication abilities emerge in developing children.

Are only infants held more often on the left? If so, why? Testing the attention-emotion hypothesis with an infant, a vase, and two chimeric tests, one "emotional," one not

Most adults, especially women, hold infants and dolls but not books or packages on the left side. One reason may be that attention is more often leftward in response to infants, unlike emotionally neutral objects like books and packages. Women's stronger bias may reflect greater responsiveness to infants. Previously, we tested the attention hypothesis by comparing women's side-of-hold of a doll, book, and package with direction-of-attention on the Chimeric Faces Test (CFT) [Harris, L. J., Cárdenas, R. A., Spradlin, Jr., M. P., & Almerigi, J. B. (2010). Why are infants held on the left? A test of the attention hypothesis with a doll, a book, and a bag. Laterality: Asymmetries of Body, Brain and Cognition, 15(5), 548-571. doi:10.1080/13576500903064018]. Only the doll was held more often to the left, and only for the doll were side-of-hold and CFT scores related, with left-holders showing a stronger left-attention bias than right-holders. In the current study, we tested men and women with a doll and the CFT along with a vase as a neutral object and a "non-emotional" chimeric test. Again, only the doll was held more often to the left, but now, although both chimeric tests showed left-attention biases, scores were unrelated to side-of-hold. Nor were there sex differences. The results support left-hold selectivity but not the attention hypothesis, with or without the element of emotion. They also raise questions about the contribution of sex-of-holder. We conclude with suggestions for addressing these issues.

Forelimb preferences in quadrupedal marsupials and their implications for laterality evolution in mammals

BACKGROUND

Acquisition of upright posture in evolution has been argued to facilitate manual laterality in primates. Owing to the high variety of postural habits marsupials can serve as a suitable model to test whether the species-typical body posture shapes forelimb preferences in non-primates or this phenomenon emerged only in the course of primate evolution. In the present study we aimed to explore manual laterality in marsupial quadrupeds and compare them with the results in the previously studied bipedal species. Forelimb preferences were assessed in captive grey short-tailed opossum (Monodelphis domestica) and sugar glider (Petaurus breviceps) in four different types of unimanual behaviour per species, which was not artificially evoked. We examined the possible effects of sex, age and task, because these factors have been reported to affect motor laterality in placental mammals.

RESULTS

In both species the direction of forelimb preferences was strongly sex-related. Male grey short-tailed opossums showed right-forelimb preference in most of the observed unimanual behaviours, while male sugar gliders displayed only a slight, not significant rightward tendency. In contrast, females in both species exhibited consistent group-level preference of the left forelimb. We failed to reveal significant differences in manual preferences between tasks of potentially differing complexity: reaching a stable food item and catching live insects, as well as between the body support and food manipulation. No influence of subjects' age on limb preferences was found.

CONCLUSIONS

The direction of sex-related differences in the manual preferences found in quadrupedal marsupials seems to be not typical for placental mammals. We suggest that the alternative way of interhemispheric connection in absence of corpus callosum may result in a fundamentally distinct mechanism of sex effect on limb preferences in marsupials compared to placentals. Our data confirm the idea that non-primate mammals differ from primates in sensitivity to task complexity. Comparison of marsupial species studied to date indicate that the vertical body orientation and the bipedalism favor the expression of individual- and population-level forelimb preferences in marsupials much like it does in primates. Our findings give the first evidence for the effect of species-typical posture on the manual laterality in non-primate mammals.

Differential effect of ill-being and chronic stress on cradling behavior of first and multi-time parents

A preference for cradling infants on the left side has been demonstrated in women, as has a relation of reduced left-cradling and stress/negative affect. This relation has not yet been investigated in male participants. Due to less left-cradling in non-fathers compared to fathers it was suggested that fatherhood might have an influence on cradling behavior. The present study investigated the cradling preference of first- and multi-time parents before and after birth, and its relation to ill-being and stress. Results revealed that cradling behavior of first-time fathers was not different before and after the birth of the infant. Thus, fatherhood does not seem to have an acute influence on cradling behavior. Furthermore, cradling behavior of first- and multi-time parents was differentially influenced by ill-being and stress. These results present new information about the course of cradling preference from pregnancy to postpartum and indicate that the relation of cradling and ill-being/stress is more complex for parents than for non-parents.

Why are infants held on the left? A test of the attention hypothesis with a doll, a book, and a bag

Most adults, especially women, hold infants and objects representing infants, such as dolls, preferentially on the left side. The attention hypothesis credits the effect to left-directed attention for perception of emotionally salient targets, faces being prime examples. Support comes from studies showing stronger left visual hemispace (LVH) biases in left-holders than right-holders on the Chimeric Faces Test (CFT), but control tests with non-social/emotional objects are needed. We therefore observed young women holding a doll, a book, and a bag, and compared their scores with their performance on the CFT. We also assessed their handedness to check on its possible role. Overall, only the doll elicited a significant side bias, with 57% of all holds on the left, 2% in the middle, and 41% on the right. On the CFT, only left-holders had an LVH bias, whereas right-holders had no bias in either direction. Only the doll-hold scores were consistently related to CFT scores, and for none of the objects was handedness related to side-of-hold.

Cold pressor stress reduces left cradling preference in nulliparous human females

The left cradling preference refers to the finding that women hold their infants more frequently on the left side of their own bodies. Several observational studies showed reduced left cradling during stressful circumstances, such as mother-infant separation, or domestic violence. However, until now no experimental study was conducted to investigate the immediate impact of stress on cradling behaviour. Half of the 64 female subjects participating were randomly assigned to a stressful bilateral cold pressor test. The remaining subjects performed a non-stressful control procedure. Before and after this intervention, cradling behaviour was assessed using a baby-like doll. Subjects showed a left cradling preference prior to the intervention. The cold pressor test increased blood pressure and heart rate significantly. A repeated ANOVA revealed an interaction of intervention (cold pressor vs. control) X assessment period (pre- vs. post-intervention), indicating that cold pressor stress reduces left cradling behaviour in female volunteers. Our data indicate that stress influences cradling preference. This may be of relevance for caregiver-infant interactions.

Mothers’ and fathers’ lateral biases for holding their newborn infants: A study of images from the World Wide Web

Photographic and direct-observation studies show that most adults hold infants on the left side. This basic directional effect is well established, but other details are still uncorroborated, uncertain, or inconsistent across studies. These include the overall strength of the bias, the role of the sex, parental status, and experience of the holder, and the sex and age of the infant. Given their importance for understanding the bias, we sought further information from a large sample of photographs of mothers and fathers, some of them first-time parents, others not, holding their infants in the first minutes, hours, or days after birth. The results confirmed the basic directional effect and provided information on the other variables. They also raise questions for further research, especially as it pertains to the use of photographs vs direct observation.

Infant holding biases and their relations to hemispheric specializations for perceiving facial emotions

Since Salk [Salk, L. (1960). The effects of the normal heartbeat sound on the behavior of the new-born infant: implications for mental health. World Mental Health 12, 168-175] reported a left-side preference for cradling an infant, several studies have attempted to elucidate the origin of this bias. Sex and handedness were the first variables tested but none of them is sufficient for explaining this bias. Manning and Chamberlain [Manning, J. T., & Chamberlain, A. T. (1991). Left-side cradling and brain lateralization. Ethology and Sociobiology, 12, 237-244] proposed that the explanation had to do with hemispheric specialization of emotions and suggested that the mother could better monitor her infant's emotional state when holding on the left side than on the right side. Moreover, the infant could monitor its mother's emotional state, since the most expressive side of mother's face (the left) is visible to the infant. We used two Chimeric Figures Tasks in order to assess (1) the preferred visual field for perceiving an emotion and (2) the most expressive side of the face. Holding biases were measured in a concrete situation using an infant doll. Our main objective was to assess the relation between the asymmetric visual perception and the holding direction in a large sample of university students. We replicated a left-holding preference (66%) in our sample and found an effect of participants' holding posture and a limited effect of laterality but no effect of sex. The most significant finding concerns the links between the preferred visual field and the preferred holding side. This effect was observed in the sample of women, in right-handers, and in the sub-group of participants with care-giving skills. These findings suggest a leading role for the right hemisphere for side of holding.

Maternal cradling and infant nipple preferences in rhesus monkeys (Macaca mulatta)

This study investigated lateral biases in nipple preferences, maternal cradling, carrying, and retrieval in 41 rhesus macaque (Macaca mulatta) mother-infant dyads living in two captive social groups. Observations were made during the first 6 weeks of infant life using a combination of scan sampling and ad-libitum sampling techniques. Infants exhibited a significant left-nipple preference in the first weeks of life but the bias decreased with infant age. Mothers showed a left-arm bias in carrying their infants but no significant lateral bias in cradling or retrieval. Our results suggest that the left-side cradling bias reported in studies of humans and some other primates reflects a bias in the infant's nipple preference rather than in maternal behavior. The infants' preference for the left nipple is consistent with both Salk's (1960) heartbeat hypothesis and with more recent hypotheses linking this lateral bias with brain asymmetry and hemispheric specialization for mother-infant communication.

A child-holding thought experiment: students prefer to imaging holding an infant on the left side of the body

The tendency of women to prefer to hold infants on the left side of the body has been confirmed by several methods: surveys of works of art and of photographs, field observations, and experimental methods. The present study developed a new method to examine the patterns of child-holding, that is, the child-holding thought experiment. 2287 female and 1268 male undergraduates were instructed to imaging nursing or feeding a baby. Then, they were asked on which side of the body they imagined holding the baby. Seventy-two percent of the women and 65% of the men reported imaging holding a baby on the left side of the body. This preference for the left side was noted, irrespective of handedness. These results indicate that both women and men who have never fed their own babies prefer to imaging holding a baby on the left side of the body. Moreover, that the magnitude of this preference was significantly stronger for the women adds to the evidence of a gender-dependent expression for lateral preferences in holding a baby.