Father-Inclusive Parenting Intervention in Mwanza, Tanzania

Maternal and paternal stimulation practices will be measured by the Family Care Indicators (FCI), which are reported in terms of the number of developmentally-enriching activities that the parent engaged with the young child in the past week (e.g., singing, telling stories). Activities adapted from the Family Care Indicators. The total score ranges from 0 to 7, with higher scores indicating greater parental engagement in stimulation activities.

Early child development will be measured using the Caregiver Reported Early Development Instrument (CREDI) long form. It relies on parental reporting and assesses four domains of early developmental milestones: motor, language, cognitive, and socioemotional development. Higher scores reflect a greater number of age-appropriate developmental skills as reported by caregivers.

Play and learning materials will be measured using the Early Childhood Development Module of the Multiple Indicator Cluster Survey. Includes types of different objects children play with at home (e.g., homemade toys, manufactured toys, household objects) as well as the number of books present in the household. Play and learning modules will be reported by the mother.

Maternal and paternal disciplinary practices will be reported in terms of harsh punishment (e.g., shook the child, shouted at the child) and positive disciplinary practices (e.g., explained why the behavior was wrong) used by the parent towards the child. Items were adapted from the Child Discipline Module of the Multiple Indicator Cluster Survey, with additional items related to verbal harsh punishment and positive reinforcement created by the study team based on formative research within this study context. Separate indicators will be created for the use of any harsh disciplinary practices or any positive disciplinary practices. Higher values on each indicator will reflect greater use of the respective disciplinary practice (i.e., harsh or positive).

The level of the fathers' involvement will be self-reported by men as well as through maternal reports about their male partners regarding the number of childcare and household activities performed by the father to support the young child, female partner, and family more broadly (e.g., bathing the child, providing money to the partner, washing dishes/clothes). Measure developed and validated by the authors. Higher scores indicate greater fathers' involvement in childcare activities and household chores.

Attitudes toward parenting will be reported by mothers and fathers using a tool developed by the study team, informed by formative qualitative research conducted in the study context. The tool captures agreement with statements related to developmentally appropriate parenting, general parenting practices, and discipline approaches. Higher scores reflect more positive attitudes toward parenting.

Gender attitudes reported by mothers and fathers regarding gender norms around childcare and domestic responsibilities, household equity, and power dynamics within partner relationships. This measure was developed by the study team based on formative research in this context. Higher scores indicate more gender-equitable attitudes

Mothers and fathers will report on who in their family (mother, father, someone else, or shared) has the final say about decisions across various family domains. Items were adapted from the decision-making module of the Demographic Health Surveys. Women's decision-making autonomy will be scored as the number of domains in which she is involved, with higher scores indicating greater participation.

Parenting distress will be reported by mothers and fathers using the Parenting Stress Index-Short Form, Parental Distress subscale. The scale is based on a series of statements that parents can agree or disagree with, and the scores are calculated by summing the responses to the items. Higher scores indicate greater levels of stress.

Mothers will report on their experiences of intimate partner violence in the past three months using a standardized set of violent acts perpetrated by their partner (i.e., the child's father), adapted from the World Health Organization's multi-country study on women's health and domestic violence against women. The primary IPV indicator will be a proportion reflecting the percentage of mothers who report experiencing any form of IPV. In addition, we will calculate separate binary indicators for each type of IPV (physical, sexual, and emotional).

Child dietary diversity will be reported by mothers as the number of food groups (out of eight) consumed by the child in the previous 24 hours, following World Health Organization (WHO) guidelines. This measure is assessed in children aged six months and older, with higher scores indicating a greater likelihood of achieving a nutritionally adequate diet.

Methylation pace of aging was developed from DNA-methylation analysis of Pace of Aging in the Dunedin Study birth cohort (DunedinPACE). Pace of Aging is a composite phenotype derived from analysis of longitudinal change in 18 biomarkers of organ-system integrity (Belsky et al., 2015). Increments of methylation pace of aging correspond to "years" of physiological change occurring per 12-months of chronological time.

Salivary DNA-methylation profiles of cognitive functioning, i.e., "Epigenetic-g", can be computed on the basis of weights from a blood-based epigenome wide association study of general cognitive functions (g) in adults. General cognitive ability was derived from the first unrotated principal component of logical memory, verbal fluency and digit symbol tests, and vocabulary. Epigenetic-g is conceptually distinct from biological aging. If a higher quality measure of epigenetic profile of cognitive functioning becomes available at the time of analysis, we will substitute that instead.