Starting Age Overshadowed: The Primacy of Differential Environmental and Family Support Effects on Second Language Attainment in an Instructional Context

636 secondary-school students

Swiss secondary-school students who had all learned Standard German and French at primary school; half had learned English from age 8, half started English five years later

Five-year longitudinal study using a natural policy contrast between Swiss cantons with different English starting ages. Not randomised, and cantons differ in more than starting age. The design's strength is that it tracks the same students for five years and measures family context and biliteracy directly, so it can ask WHO benefits from an early start rather than only whether the average student does.

Age-related attainment effects are overshadowed by other effects. An earlier age of learning proved beneficial ONLY for children raised as biliterate simultaneous bilinguals receiving substantial parental support; for monolinguals and for non-biliterate bilinguals (simultaneous or sequential) the early start conferred no advantage. This is a moderation result, and the moderator is family support and existing biliteracy - which is exactly the variable this archive treats as presumptively confounded with heritable ability.

High for the moderation claim specifically. "Biliterate simultaneous bilingual children with substantial parental support" is a family-selected category, not an assigned condition; the finding that they alone benefit from an earlier start is a correlation between a heritable and environmentally advantaged trait and a response to treatment, not an established aptitude-treatment interaction.

Consistent in direction with the Barcelona Age Factor project and with Jaekel et al. (2017): in instructed foreign-language settings, an earlier start does not produce a durable advantage at equal hours. The specific moderation by family biliteracy is unreplicated.