Effects of intergenerational contact on social capital in community-dwelling adults aged 25–84 years: a non-randomized community-based intervention

BACKGROUND: Accumulating social capital in urban areas is essential to improve community health. Previous studies suggested that intergenerational contact may be effective for enhancing social capital. However, no study has examined the effect of intergenerational contact on social capital through a population-based evaluation. This study aimed to investigate the effects of a community-based intervention to increase the frequency of intergenerational contact on social capital among adults aged 25–84 years. METHODS: This study used a non-randomized controlled trial design to conduct a community-based intervention (from March 2016 to March 2019). The study area was Tama ward, Kawasaki city, Kanagawa, Japan. The area comprises five districts; one district was assigned as the intervention group and the other four districts as the control group. We provided the intervention to residents in the intervention group. The intervention comprised three phases: Phase 1 was the preparation term (organizing the project committee); Phase 2 was the implementation term (trained volunteer staff members, conducted the intergenerational greeting campaign, and held intergenerational contact events); and Phase 3 was the transition term (surrendering the lead role of the project to the city hall field workers). In the control group, field workers provided public health services as usual. We conducted mail surveys in September 2016 and November 2018 to assess the effects of the intervention on social capital during Phase 2. Eligible participants were randomly selected from community-dwelling adults aged 25–84 years according to age (10,620 control group individuals and 4479 intervention group individuals). We evaluated social trust, norm of reciprocity, and social support as outcome variables. RESULTS: In total, 2518 participants completed both surveys and were analyzed (control group: 1727; intervention group: 791). We found that social trust (coefficient = 0.065; 95% confidence interval [CI]: 0.006, 0.125) and norm of reciprocity (coefficient = 0.084; 95% CI: 0.020, 0.149) positively changed in the intervention group compared with the control group. CONCLUSIONS: This community-based intervention may contribute to sustaining and improving social capital among community-dwelling adults. Trial registration: UMIN000046769 (UMIN-CTR); first registered on January 28, 2022 (retrospectively registered). SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12889-022-14205-6.