Introduction

Digital media, including on mobile phones, tablets, and PCs, is frequently utilized by many mothers. Average daily mobile phone use by mothers is reported to be 1.65 h in Germany, 3.17 h in Thailand, 2.67 h in South Korea, and 2.23 h in Japan1,2,3,4. Digital media offers several benefits for mothers of young children, such as access to child-rearing information, opportunities for interaction, and means to alleviate stress and boredom3,5. However, there are associations between digital media use and mental health issues such as depression, anxiety, stress, low self-esteem, loneliness within relationships, low emotional regulation and mindfulness, attachment challenges, and negative parenting5,6,7,8,9,10. Moreover, the use of digital media by mothers is linked to reduced verbal and non-verbal interactions with, and responsiveness to, their children11,12,13. Excessive smartphone dependence may result in mother–child attachment disorders, maternal neglect, and increased aggression toward children3,8. High media usage by mothers also correlates with emotional and conduct problems, as well as hyperactivity and inattention, in children1. Additionally, maternal smartphone dependency is associated with aggression, oppositional behavior, and emotional instability in children, thus leading to negative developmental tendencies such as impaired self-expression and self-regulation3.

Previous studies have reported no association between maternal digital media use time and children’s developmental outcomes2,14. In contrast, one study examined the association between the likelihood of parents using digital media during common everyday routines with their children and children’s language development15. Although this study found no association between parents’ digital media use and children’s language development, it did reveal a high likelihood that parents’ use of digital media during daily routines with their children is negatively correlated with children’s language development15. Previous research on the association between mothers’ digital media use time and children’s developmental outcomes has focused on the daily duration of mothers’ digital media use, rather than limiting focus to time spent with children2,14,15. Therefore, the results may be different if we examine the association between mothers’ digital media use and children’s developmental outcomes when we focus only on the mothers’ digital media use while with their children. Previous studies have shown that quality early mother–infant interactions, especially maternal responsiveness, are critical predictors of a child’s social, emotional, cognitive, and linguistic development11,16,17,18. Parental smartphone use reduces parental sensitivity and responsiveness, negatively impacting parent–child interactions19. The frequent interruption of interpersonal relationships and shared time by the use of technology devices—referred to as “technoference”20,21—has been suggested to hinder word learning and interaction outcomes22. Therefore, it is important to consider the association between mothers’ digital media use and children’s development by focusing on mothers’ time spent with their children, when their interactions could be interrupted.

In this study, in addition to the mother’s mental health (neuropsychiatric disorders, psychological distress, bonding failure), which is related to digital media use as mentioned above, we also included the child-rearing environment (e.g., frequency of reading to the child, number of hours per day the child is allowed to watch TV or DVDs) and socioeconomic status as adjustment variables. No previous studies have examined the association between digital media use and child development using multiple adjustment variables. Furthermore, no objective evaluations have been reported on the association between mothers’ use of digital media and their children’s developmental outcomes in Japan.

The Japan Environment and Children’s Study (JECS)—a large-scale birth cohort survey—has been conducted since 2011, targeting 100,000 pairs of mothers and children nationwide in Japan23,24. Trained inspectors utilize the Kyoto Scale of Psychological Development 2001 (KSPD) to assess the development of two-year-old children in a Sub-Cohort Study of 5% of JECS participants, selected by random sampling25.

The first 1,000 days from conception to age two are deemed crucial for establishing the physical, cognitive, and socioemotional foundations for subsequent life, as the period with the greatest brain plasticity26. During this time, the brain develops the most rapidly and can be greatly influenced by nutrition, protection (e.g., from toxic stress, violence, abuse, and neglect), and responsive stimulation (talk, play, reading, and responsive interaction with a loving adult)27. Additionally, the developmental status at age two can predict later school performance and socioeconomic status in adulthood28.Therefore, the age of two is an important milestone for assessing development and the nurturing environment of the mother and child.

The aim of this study was to investigate the association between the amount of time mothers spent using digital media while with their children and their children’s developmental outcomes at age two, as assessed by the KSPD, for a large cohort. The developmental outcomes used were motor, cognitive, and language development, as well as overall development, which included all three outcomes. In addition, mother’s mental health, child-rearing environment, and socioeconomic status were used as control variables.

Materials and methods

JECS

The JECS aims to elucidate the effects of environmental factors on children’s growth and development. The protocol for this study has been published in extant studies23,24. From January 2011 to March 2014, participants were recruited from 15 Regional Centers nationwide; 100,000 pairs of mothers and children were registered and will be followed up until the children turn 13.

A sub-cohort study was planned for 5% of the 100,000 JECS participants through random sampling to obtain more detailed information via an objective evaluation method (such as medical examinations by pediatricians and psychological development assessments by trained staff)25.

The JECS was conducted in accordance with the guidelines of the Declaration of Helsinki. The JECS protocol was reviewed and approved by the Ministry of the Environment’s Institutional Review Boards on Epidemiological Studies (No. 100910001) and the Ethics Committees of all participating institutions. Written informed consent was obtained from all of the participants or their legal guardians.

Design and participants

For our study, we adopted data on the participants included in the JECS Sub-Cohort Study. We used dataset jecs-ta-20190930, which was issued in October 2019 and revised in November 2022. The dataset contains 104,062 pairs of parent–child data points (including miscarriages and stillbirths) collected from gestation to age three. In a sub-cohort study, the KSPD was conducted at two years of age, providing data on 4,988 pairs. The exclusion criteria were invalid test results, multiple births, and missing exposure or covariate data (Fig. 1). As a result, we targeted 3,786 pairs of parents and children.

Fig. 1
Fig. 1
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Flow chart for the selected participants. We used data from 104,062 pairs of pregnant women and fetuses, including multiple births. Of these, the Kyoto Scale of Psychological Development 2001 (KSPD) was administered to participants in the sub-cohort at age two years, yielding data on 4,988 pairs. After excluding invalid KSPD results, multiple births, and those with missing data related to exposure and covariates, we finally analyzed 3,786 pairs.

Exposure: Mothers’ digital media use duration

To investigate the amount of time mothers spent using digital media while with their children, we used responses from a self-administered parental questionnaire mailed to caregivers when the child was one year and 11 months old. The questionnaire inquired about the number of hours per day the respondents spent on the computer, mobile phone, tablet, or video games while with their children. Respondents were asked to choose from “none”, “less than one hour”, “one hour or more but less than two hours”, “two hours or more but less than four hours”, and “four hours or more”. Using these caregiver responses and referring to previous studies15, the responses “none”, “less than one hour”, “one hour or more but less than two hours”, and “two hours or more” were classified into four groups. Unanswered exposure questions and non-mother responses were excluded due to being considered missing exposure data.

Outcomes: Children’s motor, cognitive, language and overall development

Children’s developmental outcomes were assessed using the KSPD, which evaluates three domains: postural-motor (P-M), cognitive-adaptive (C-A), and language-social (L-S). P-M corresponds to fine and gross motor abilities; C-A to non-verbal cognitive capacity and visual-spatial comprehension; and L-S to interpersonal relationships, socialization, and verbal abilities29. The KSPD is an individual, standardized, and developmental face-to-face test that contains 328 items for Japanese children and adults30. In this study, the KSPD was conducted only with children. Based on the KSPD manual, the scores for these three areas and total scores were converted to developmental ages using a conversion table. In addition, developmental age was divided by the corrected chronological age (chronological age corrected based on gestational day 280, considering the influence of gestational age) and multiplied by 100 to calculate the developmental quotient (DQ)30,31. Thus, DQs (continuous variables) for P-M, C-A, L-S, and the total were calculated.

The KSPD has been compared to other face-to-face developmental tests, such as the Bayley Scales of Infant and Toddler Development 3rd Edition (Bayley-III) and Denver Developmental Screening Test II (DDST-II). The relationships between the corresponding pairs of KSPD domains and Bayley-III scores were analyzed and correlations were found: C-A DQ and cognitive composite scores, L-S DQ and language composite scores, and P-M DQ and motor composite scores31. The correlation between the KSPD DQ and the DDST-II has been analyzed, and correlations between the P-M DQ and gross motor, C-A DQ and fine motor-adaptive/personal-social, and L-S DQ and language have been confirmed32. The KSPD DQ had a significant positive correlation with IQ on the Tanaka-Binet Intelligence Scale (Japanese version of the Stanford-Binet Intelligence Scale); the total DQ had the highest correlation29.

As part of the JECS, the KSPD is used to assess two-year-olds (ranging from one month before to three months after they turn two). Certified inspectors conduct the KSPD following standardized procedures and quality control measures33,34.

Statistical analysis and covariables

The characteristics of the participants were analyzed using descriptive statistics. Using the χ2 and t-tests, we examined the differences in mothers’ duration of digital media use and KSPD DQ by children’s sex. Finally, we examined the association between mothers’ digital media use and the development of their two-year-old children using multiple regression analysis, and conducted stratification analysis by children’s sex. In the multiple regression analysis, we adjusted for various covariates that could affect maternal digital media duration and child development: maternal age at delivery (continuous variable); marital status (married, common law marriage/divorced, widowed, other); maternal psychological distress (The 6-item Kessler Psychological Distress Scale [K6] ≧5: no/yes)35; neuropsychiatric disorders in mothers (no/yes); maternal bonding failure (The Japanese version of Mother-to-Infant Bonding Scale [MIBS-J] ≧5: no/yes)36; maternal education level (< 10; 10 to < 13; ≧13); paternal education level (< 10; 10 to < 13; ≧13); maternal occupation (no/yes); household income (× 10^3 yen/year; < 2,000; 2,000 to < 4,000; 4,000 to < 6,000; ≧6,000); family structure (nuclear/extended); partner’s frequency of participating in child care activities (never, seldom/sometimes/often, very often); frequency of meeting with other parents of same-aged children (seldom, 1–3 times a month, 1–2 times a week, 3–4 or more times a week); number of children including the participant (1, 2, ≧3); presence of physical anomalies (no/yes)37; attendance at a daycare center or nursery (no/yes); frequency of reading to the child (1–3 times a month or less, 1–2 times a week, 3–4 times a week, 5 or more times a week); number of hours per day the child is allowed to watch TV or DVDs (< 1, 1 to < 2, ≧2); and outdoor playtime during summer (May to September) and winter (November to February) (hours per day) (< 1, ≧1). Data on these covariates were gathered through responses to a self-report questionnaire and from medical record transcripts. Statistical significance was set at p < 0.05. All statistical analyses were conducted using SPSS 28.0 for Windows (SPSS Inc., Chicago, IL, USA).

Results

Table 1 presents the characteristics of the 3,786 parent–child pairs included in the study. Regarding the daily duration of mothers’ digital media use while with their children, 15.4% reported “none”, while 63.2%, 16.5%, and 4.9% reported “less than one hour”, “one hour or more but less than two hours”, and “two hours or more”, respectively. The results of the χ2 test indicated no significant differences in the duration of mothers’ digital media use based on the sex of the child (χ2 (3) = 2.30, ns).

Table 1 The characteristics of the participants (N = 3,786).
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The average (SD) postural-motor (P-M), cognitive-adaptive (C-A), language-social DQs, and total DQ were 94 (18.0), 96 (12.6), 92 (14.9), and 94 (10.4), respectively. The results of the t-test showed significant differences in DQ and sex (P-M DQ (t(3784) = 2.51, p = 0.012), C-A DQ (t(3784) = 10.47, p < 0.001), L-S DQ (t(3784) = 12.93, p < 0.001), and total DQ (t(3784) = 12.55, p < 0.001)). P-M, C-A, L-S and total DQ were higher in girls than in boys. In this study, girls had a more advanced development than boys at the age of two years.

Table 2 presents the results of the multiple regression analysis of mothers’ digital media use duration and DQ after adjusting for covariates. No multicollinearity was detected in this analysis (variance inflation factor (VIF) < 3). Compared with the “none” group, the “one hour or more but less than two hours” group (B = −3.07; 95% CI −4.81 to −1.33) and the “two hours or more” group (B = −5.24; 95% CI −7.76 to −2.73) exhibited lower L-S DQs. Additionally, compared with the “none” group, the total DQ was lower for the “two hours or more” group (B = −2.07; 95% CI −3.86 to −0.28).

Table 2 Multiple regression analysis of mothers’ digital media use duration and KSPD scores (N = 3,786).
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The results of the analysis stratified by children’s sex revealed that, for both boys and girls, compared with the “none” group, the “one hour or more but less than two hours” (Male B = −2.64; 95% CI −5.01 to −0.27, Female B = −4.12; 95% CI −6.57 to −1.67) and the “two hours or more” (Male B = −5.21; 95% CI −8.64 to −1.79, Female B = −5.10; 95% CI −8.66 to −1.55) groups had lower L-S DQs.

Discussion

This study investigated the association between the amount of time mothers spent using digital media while with their children and their children’s developmental outcomes at age two (motor, cognitive, language and overall development), as assessed by the KSPD, for a large cohort. In this study, most mothers (78.6%) reported using digital media for less than one hour per day when they were with their children, while 21.4% spent more than one hour on digital media. When mothers used digital media for more than one hour while with their children, the L-S DQs were lower, indicating a dose–response relationship. Additionally, the total DQs were lower when mothers’ digital media use extended to two hours or more.

This study showed that when mothers spent more than two hours a day using digital media while with their children, their total DQ score decreased and was more negatively associated with children’s development. The total DQ is related to the full-scale intelligence quotient of the Wechsler Intelligence Scale for Children-III, which is administered to children aged five and above38. Although the difference in total DQ was slight, the long-term effects of mothers’ continuous digital media use on children’s subsequent development remain unclear. This study found no association between maternal digital media use and children’s motor or cognitive development. However, previous studies have shown that there is a positive correlation between maternal digital media use and the duration of children’s digital media use, as well as a negative correlation between children’s digital media use and their cognitive development39,40,41. Furthermore, previous research also revealed a positive correlation between the amount of time children spend using digital media and delays in gross motor development2. Although the impact of children’s digital media usage time could not be adjusted for in this study, the impact of TV, which is the digital device most frequently used by this age group, was adjusted for as a covariate14,15,42. This study examined the association between mothers’ duration of digital media use while with their children and their children’s motor and cognitive development, while adjusting for children’s screen time as a covariate, and found no association.

Maternal digital media use was also found to be negatively associated with children’s language development. Previous studies reported no association between mothers’ digital media use duration and children’s development2,14,15. However, this study yielded different results by focusing on the amount of time mothers spent on digital media while with their children. In this study, digital media use for less than one hour per day was not negatively associated with language development. By contrast, maternal digital media use for more than one hour per day was found to be negatively associated with children’s language development. Key factors in word learning include contiguity (temporal connectedness) and contingency (contextual relevance) with the child’s utterances43,44. The qualitative aspects of children’s language experiences, such as turn-taking, may have a greater impact on language development than the quantitative aspects, such as adult word count43,45. The problematic use of digital media by parents and “technoference” in parent–child interactions are associated21. In a previous Swedish study, 75% of parents of two-year-old children reported using their smartphones at least some of the time during playtime15. In a previous study in the United States, 11% of parents reported that on a typical day, they had no technoference in their interactions with their children, while many other parents reported one or more instances of technoference21. Parental technoference can adversely affect interactions with children of all ages by diminishing parental attention, responsiveness, and warmth21,46. Maternal smartphone use decreases the quality and frequency of interactions with the child and may adversely affect the child’s development of language, cognition, and socioemotional regulation47.

While there are negative impacts of parental digital media use on interactions with children, there are also benefits to using digital media. Previous research indicated that parent–child verbal interactions during media use may mitigate the negative impacts on language development, while interactions during educational content may directly enhance language skills48. Additionally, digital media offers benefits for the parents of young children, who may use mobile devices to communicate with others, regulate emotions, and alleviate feelings of loneliness19,49,50.

We further explored the association between the duration of mothers’ digital media use and the development of their children by sex. No differences were found in the duration of digital media use by mothers between boys and girls. Similarly, previous studies have also found no difference between mothers’ dependence or excessive use of smartphones and the sex of their young children3,4. However, we found that girls were more advanced than boys in all developmental outcomes at the age of two, including motor, cognitive, and language. A previous study also reported that girls scored higher on all subjects on the Bayley-III test administered to children aged 18–23 months51. However, reports exist that at certain ages, girls scored lower than boys51; sex differences in developmental outcomes need to be further examined over time. The sex stratification analysis revealed that mothers’ digital media use exceeding one hour was negatively correlated with children’s language development, while showing no variance between boys and girls. Future studies are needed to determine whether sex differences emerge as young children become older. Notably, when stratified by sex, mothers’ media use beyond two hours showed no association with total DQ. As only 4.9% of mothers in this study reported using digital media for more than two hours daily, the limited sample size may have reduced the power of the stratification analysis.

The quality and quantity of language exposure during childhood, a sensitive period for neuroplasticity, are foundational for subsequent verbal skills, literacy, executive function, mathematical abilities, and social competencies43. Cognitive stimulation through reading and shared play at home is deemed critical for fostering interactions48,52. In this study, the frequency of reading was controlled as a significant child-rearing environment factor. Nevertheless, maternal digital media use was shown to have a significant negative correlation with child development. Further investigation is required to understand the long-term effects of parental digital media use on children’s development beyond the age of two, including the impact of various child-rearing environments.

This study has several limitations as follows. First, we were unable to determine mothers’ purpose of digital media use and children’s situation during this time (i.e., whether they were using digital media together or were engaged in other activities). Second, we were unable to adjust for the effects of children’s digital media use duration. However, given that television is the most frequently used digital device for this age group14,15,42, its effect was adjusted for as a covariate. Third, because mothers’ duration of digital media use was self-reported in the questionnaires, we cannot rule out underreporting53. Additional studies employing objective measurement methods are thus required.

The main strength of this study lies in its investigation of the association between mothers’ digital media use duration while with their children and the children’s development, utilizing a large cohort and considering factors such as mother’s mental health, child-rearing environment, and socioeconomic status.

In conclusion, the use of digital media by mothers for more than one hour per day while with their two-year-old children is negatively associated with language development. In addition, use of more than two hours is more negatively associated with their children’s development. We recommend limiting digital media use to less than one hour per day when mothers are with their two-year-old children.