Causal diagram for the effect of familial HBV marker on hepatitis B e antigenaemia in the HBeAg Study, The Gambia, 1991–1993. Maternal HBV sero-marker and number of older siblings with positive HBV sero-marker are proxies for perinatal and early postnatal HBV transmission, respectively. Early age at HBV infection is associated with positive HBeAg through increasing the risk of chronic HBV infection. The hypothesis of this analysis is in addition to this effect, early age at infection further increases the risk of hepatitis B e antigenaemia (presented as a dashed arrow). The variables which were not measured are surrounded by dashed lines, and the exposure and outcome variables of interest are surrounded by lines in bold-type. SES denotes socio-economic status.