This study analyses differences in first language (L1) ability between students in content and language integrated learning (CLIL) and students in mainstream education (non-CLIL). The study provides new insights by using multiple methods to analyse this relationship in a Finnish secondary education context. The dissertation comprises three sub-studies: a small-scale case study (n = 4) (Article I), a study concerning L1 oral production (n = 31) (Article II), and a study focusing on L1 writing and speaking (n = 30) (Article III).

The participants in all sub-studies were either 14 or 15-year-old L1 Finnish speakers who were enrolled in either CLIL or mainstream education in Finland. The data were analysed using quantitative methods in order to assess group-level differences in L1 ability and identify correlations between L1 ability and individual differences in socioeconomic status (SES) and academic achievement in L1 as a subject. Specifically, Article I explores approaches to assessing differences in CLIL and non-CLIL students’ performance in both Finnish and English. Article II examines lexical access and retrieval in L1 oral production using a verbal fluency task and a picture-naming task. Article III analyses students’ L1 writing via a short written task based on a prompt question concerning the environment and their L1 speaking via an elicitation task based on a short comic strip.

The results suggested that CLIL students are not disadvantaged in terms of L1 ability in Finnish secondary education CLIL contexts, where L1 use is typically not avoided or forbidden. The correlation analyses also underscored the importance of accounting for SES in the analysis of L1 ability in both CLIL and mainstream education. While the non-CLIL students’ results were suggestive of a positive relationship between SES and L1 ability, the CLIL students’ results in L1 ability did not clearly reflect differences in SES in either direction. These findings add to the growing body of research suggesting that CLIL may help promote equality by mitigating the effect that SES has on the development of language skills.