Children’s home environments for physical activity and sedentary behavior varied by socioeconomic status. Children in lower SES households had significantly greater access to electronic media devices in their bedrooms but lower access to portable play equipment. Household rules around outdoor play were more restrictive in lower SES households. These differences were found across both household income and highest level of educational attainment in the household. Children’s screen time was higher in low-SES households but there were no SES differences in children’s overall or home-based MVPA or sedentary time.
The SES disparities in screen time are similar to previous studies that found inverse associations between SES and screen-based media use [19, 20]. Approximately half of the children from low SES families in this sample had a television in their bedroom and a quarter had a video game system, significantly higher than for children from high SES families. This paradox between low SES and high access to often expensive equipment has been explained by findings suggesting that parents in low SES families have greater concerns about their neighborhood’s safety , may lack time to supervise children in their neighborhoods,  and have less access to alternative activities, [23, 24] which makes indoor screen-based entertainment an appealing alternative to outdoor play. Extensive marketing of electronic entertainment devices may be another contributing factor. Furthermore, higher parental SES may be related to greater awareness of and ability to adopt screen time recommendations; supporting the theory that many initiatives intended to improve population health also may increase disparities since social position determines how well one can adopt preventive health knowledge . Our screen time results (mean of 1.9 hours/day), however, were lower than national estimates that suggest children this age are exposed to over 3–4 hours of screen time per day .
The physical activity results in this study are consistent with those of a review which found that various estimates of family SES were generally unrelated to children’s physical activity . Ferreria et al. hypothesized that since physical activity in younger children is mostly informal, it may not involve much extra financial cost. As activity levels generally decline with age and associated costs for athletic participation potentially increase, perhaps such disparities in income affect physical activity more in adolescents  and adults . Of note, family support for physical activities (watching, encouraging and providing transport to sports/physical activities) did not differ by SES in this sample.
Though sedentary behavior may displace some physical activity, it is not simply the inverse of active behavior, and sedentary time is also independently associated with poor health outcomes [8, 28]. Thus, focusing efforts on modifiable factors to both decrease sedentary behavior and increase physical activity in high risk groups is critical. Analyses identified some potentially modifiable factors in the home environment that were found to mediate the relationship between SES and screen time, a common sedentary behavior. Media in the bedroom, access to portable play equipment and joint screen time with parents are all potentially modifiable in interventions. Portable play equipment may stimulate active behaviors that are incompatible with screen time, though affordability of some equipment could be a challenge for low income families. More marketing of play equipment or counter-marketing to electronic entertainment targeted to low SES families may be required.
Joint media use has been recommended so parents can monitor their children’s television exposure, help children interpret what they see, and moderate the impact of media exposure by reducing adverse effects and increasing the possibility of benefit . However, a previous study found that co-viewing was not motivated by parental determination to mediate children’s television experiences, and it occurred less often with younger children who need it most . That study found parents co-view with children when their viewing preferences coincide, and co-viewing is associated with positive parental attitudes towards television. Thus, excessive parent–child joint screen time appears to be a risk factor for child screen time and is an under-studied correlate of child sedentary time that could be targeted in an intervention. A better understanding of how families spend time together and interventions that promote joint physical activities could be helpful.
Media in the bedroom, especially TV, may be the most important mediator identified here because it has been associated with overweight, likely for several reasons, including greater screen time,  interference with sleep, [32, 33] and increased exposure to advertising for unhealthy foods. Previous research has found that media in the bedroom mediates the relationship between SES and BMI in adolescents . Our study highlights the need to target media in the bedroom at even younger ages.
There are some study limitations that warrant consideration. First, our screen time outcome was by parent-report, which has been shown to correlate with actual viewing time,  but is still subject to social-desirability biases that may differ by SES. Second, we did not examine school and neighborhood level factors in this study, which likely vary by SES and contribute to overall physical activity and sedentary behaviors, as well as physical activity opportunities in the school and neighborhood. Third, given the cross-sectional data for this study, we were unable to evaluate causality. Fourth, there are multiple scoring decisions and sets of cut-points for accelerometer data in children (e.g. 3 METs vs. 4 METs for moderate activity), and results can change significantly if different criteria are used, making comparisons between studies difficult . We focused on the 3 MET cut-point for moderate intensity physical activity because this is the level specified in the US physical activity guidelines,  but we did analyses using 4 MET Evenson criteria as well for comparison. Fifth, we developed a novel method (using all the information available to us) for handling accelerometer wear time during sleep hours in order to minimize inflation of sedentary time. However this approach has not been validated and we may have inadvertently eliminated some wear time. Sixth, as many complex factors influence children’s activity levels, unmeasured factors that are related to both SES and activity levels likely exist. Outdoor time, in particular, has been found to be correlated with physical activity in children and was not specifically measured in our study [9, 38]. Seventh, our sample had relatively small numbers of families in the lower SES groups, and generally high levels of physical activity across SES, potentially limiting the generalizability of our findings.
Present findings are a step in understanding SES disparities in childhood obesity. The finding that low SES home environments have more electronic devices in bedrooms and fewer pieces of play equipment than in high SES homes is cause for concern. Sedentary-promoting devices in the bedroom emerged as an important mediator of the SES-sedentary behavior association. Additional research is recommended that can inform interventions to improve the healthfulness of home environments of low SES families.