Allergic diseases have increased, with earlier rises in hay fever, eczema and asthma, followed by a rise in food allergy. This pattern has been framed in relation to Strachan's hygiene hypothesis, which proposed that larger families confer protection against allergic diseases through early-life microbial exposures 1. Subsequent epidemiological work quantified the impact of shrinking family sizes on allergy prevalence 2. The surge in food allergies has been described as the ‘second wave’ of the allergy epidemic 3, although its causes remain incompletely understood 4. We hypothesised that family structure, including family size and household composition, may influence food allergy risk through caregiving and feeding practices, particularly the timing of allergenic food introduction 5, 6. We analysed data from the Japan Environment and Children's Study (JECS), a birth cohort (2011–2014). Out of 104,059 birth records, 71,315 singleton children were eligible after restricting to households with 2–8 members. Family size was defined as the number of co-resident household members at pregnancy registration, excluding the index child. Family structure was classified into three groups—nuclear, grandparent-extended and broad-extended households—with definitions provided in the Supplement. The primary outcome was caregiver-reported, physician-diagnosed food allergy at age 4 years. We examined age at first introduction of hen's egg, shrimp/crab, buckwheat and peanut, as well as commonly tolerated foods (rice, fish, soy and chicken), based on caregiver questionnaires at 1 year. Logistic regression estimated odds ratios adjusted for child, parental and household factors (Tables S1 and S2; Figures S1–S4). Food allergy prevalence showed an inverse pattern across family size (p for trend < 0.001). At age 4 years, the prevalence was 6.4% in 2-member households (n = 24,079) and 3.2% in 8-member households (n = 659). In multivariable models, larger families had lower odds of food allergy than 2-member households (adjusted p for trend = 0.004), with adjusted odds ratios ranging from 0.80 (95% CI 0.69–0.92; p = 0.002) for 3-member households to 0.40 (0.23–0.70; p = 0.002) for 8-member households (Table 1). In contrast, allergic rhinitis and atopic dermatitis did not show consistent dose-response patterns with family size (Figure S5; Table S2). Asthma increased with family size in the univariate model (p for trend < 0.001) but not after adjustment (p for trend = 0.272). Family size was also associated with selective delays in the introduction of allergenic foods. High-risk allergenic foods were introduced later in smaller families, whereas commonly tolerated foods were introduced at similar ages across family sizes (Figure 1; Table S4). This specificity suggests that family size may be associated with food allergy risk partly through behavioural pathways that differentially affect allergenic foods rather than general feeding delays. Among firstborn households (no co-resident children at registration; n = 32,619), extended family members further mitigated these delays in allergen introduction (Figure S6). These findings suggest that family structure may influence food allergy risk through caregiving and feeding practices. This behavioural perspective complements broader theories of immune regulation 3, 4 and aligns with evidence that feeding practices and diet diversity contribute to food allergy development 6. The inverse association between family size and food allergy, together with selective delays in the introduction of allergenic foods, may provide a behavioural explanation for the protective effect historically attributed to larger families. This observational study is subject to residual confounding, misclassification of caregiver-reported food allergy and recall bias. Caregiving norms in Japan, including risk-averse feeding practices for perceived allergens, may limit generalisability. Nevertheless, the birth cohort, family structure classification and availability of early feeding data support the inference that social structures and feeding cultures contribute to contemporary food allergy risk. As societies continue to shift toward smaller, nuclear households, supporting timely allergenic food introduction in firstborn children may be an important component of future primary prevention strategies for food allergy. Takayasu Nomura and Hisashi Tanida had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Takayasu Nomura, Hisashi Tanida and Yukihiro Ohya conceived the study. Sayaka Kato, Daiki Hiraoka, Yuki Ito and Michihiro Kamijima curated the data. Takayasu Nomura, Hisashi Tanida, Takeshi Inoue, Sayaka Kato, Daiki Hiraoka, Mayumi Sugiura-Ogasawara and Yukihiro Ohya contributed to methodology. Takeshi Inoue conducted the statistical analysis with additional support from Sayaka Kato. Daiki Hiraoka and Yuki Ito contributed to validation. Takayasu Nomura created the visualisations. Takayasu Nomura and Hisashi Tanida drafted the original manuscript. Mayumi Sugiura-Ogasawara, Shinji Saitoh, Michihiro Kamijima and Yukihiro Ohya critically revised the manuscript for important intellectual content. Yukihiro Ohya supervised the analysis and manuscript preparation. All authors approved the final manuscript and agree to be accountable for all aspects of the work. Takayasu Nomura serves as the primary corresponding author. Yukihiro Ohya serves as co-corresponding author and shares responsibility for communication during the peer review. We thank the JECS participants and their families for their invaluable contributions, and the JECS Group members and staff at the 15 Regional Centres for their support in data collection and management. The findings and conclusions are solely those of the authors and do not necessarily represent the official views of the Government of Japan. ChatGPT (OpenAI) was used for language editing. All AI-assisted text was reviewed and verified by the authors. This work was supported by the Ministry of the Environment, Government of Japan. The JECS protocol was reviewed and approved by the Ministry of the Environment's Institutional Review Board on Epidemiological Studies (Approval No. 100910001) and by the ethics committees of all participating institutions. Written informed consent was obtained from all participants. The study adhered to the principles of the Declaration of Helsinki. The authors declare no conflicts of interest. Members of the JECS Group as of 2025: Michihiro Kamijima (principal investigator, Nagoya City University, Nagoya, Japan), Shin Yamazaki (National Institute for Environmental Studies, Tsukuba, Japan), Maki Fukami (National Center for Child Health and Development, Tokyo, Japan), Reiko Kishi (Hokkaido University, Sapporo, Japan), Chiharu Ota (Tohoku University, Sendai, Japan), Koichi Hashimoto (Fukushima Medical University, Fukushima, Japan), Kenichi Sakurai (Chiba University, Chiba, Japan), Shuichi Ito (Yokohama City University, Yokohama, Japan), Ryoji Shinohara (University of Yamanashi, Chuo, Japan), Hidekuni Inadera (University of Toyama, Toyama, Japan), Takeo Nakayama (Kyoto University, Kyoto, Japan), Ryo Kawasaki (Osaka University, Suita, Japan), Yasuhiro Takeshima (Hyogo Medical University, Nishinomiya, Japan), Hideki Nagashima (Tottori University, Yonago, Japan), Narufumi Suganuma (Kochi University, Nankoku, Japan), Mayumi Tsuji (University of Occupational and Environmental Health, Japan) and Kimitoshi Nakamura (Kumamoto University, Kumamoto, Japan). Data are unsuitable for public deposition due to ethical restrictions and legal framework of Japan. It is prohibited by the Act on the Protection of Personal Information (Act No. 57 of 30 May 2003, amendment on 9 September 2015) to publicly deposit the data containing personal information. Ethical Guidelines for Medical and Health Research Involving Human Subjects enforced by the Japan Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Health, Labour and Welfare also restrict the open sharing of the epidemiologic data. All inquiries about access to data should be sent to: email protected. The person responsible for handling enquiries sent to this email address is Dr. Shoji F. Nakayama, JECS Programme Office, National Institute for Environmental Studies. Data S1: Supporting Information Figures and Tables. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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