Family and intergenerational relationships are becoming increasingly important as sources of support and care for the elderly population in the rapidly aging Asian societies. However, this has also raised concern over the reinforcement of cultural preferences for sons as a source of old-age security. This paper, therefore, revisits the determinants of happiness in old age by investigating the role of adult children's gender in the context of Thailand, an aging Asian country with no legacy of sex preference in fertility. We employ nationally representative data to examine the association between old-age happiness and the presence of a coresiding child. Compared with living alone, living with at least one child is found to be positively associated with older persons' happiness. However, this result is specific to daughters. Moreover, compared with older men, older women systematically benefit from a "daughter effect." Coresiding daughters with a university education and those who maintain a good relationship with their parents help explain the positive happiness effect on older persons. Coresiding daughters are shown to increase the happiness of their parents through three channels: reducing loneliness, improving self-rated health, and improving the economic conditions of older parents. Overall, the findings of our study suggest a "daughter dividend," or access to daughters, is key to enhancing parents' happiness in Thailand. Therefore, policies that increase the human capital of girl children and enhance family solidarity are likely to have long-term intergenerational wellbeing benefits.