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This study explores how researchers’ analytical choices affect the reliability of scientific findings. Most discussions of reliability problems in science focus on systematic biases. We broaden the lens to emphasize the idiosyncrasy of conscious and unconscious decisions that researchers make during data analysis. We coordinated 161 researchers in 73 research teams and observed their research decisions as they used the same data to independently test the same prominent social science hypothesis: that greater immigration reduces support for social policies among the public. In this typical case of social science research, research teams reported both widely diverging numerical findings and substantive conclusions despite identical start conditions. Researchers’ expertise, prior beliefs, and expectations barely predict the wide variation in research outcomes. More than 95% of the total variance in numerical results remains unexplained even after qualitative coding of all identifiable decisions in each team’s workflow. This reveals a universe of uncertainty that remains hidden when considering a single study in isolation. The idiosyncratic nature of how researchers’ results and conclusions varied is a previously underappreciated explanation for why many scientific hypotheses remain contested. These results call for greater epistemic humility and clarity in reporting scientific findings.

This article examines the relationship between ethnic classroom composition and students’ language-related achievement. We investigate whether minority language use among classmates accounts for effects of ethnic composition on minority students’ German reading comprehension and their proficiency in the minority languages Russian and Turkish. We employ multilevel models using cross-sectional data from a sample of ninth-grade students participating in the German National Educational Panel Study. Our findings indicate that students’ minority language background rather than their ethnic origin accounts for ethnic composition effects. We find a negative relationship between the ethno-lingual classroom composition and students’ German reading comprehension, but the association is small and limited to minority students. Moreover, the ethno-lingual classroom composition is positively correlated with minority language proficiency, specifically among Turkish-speaking students. These associations are substantially reduced after controlling for students’ minority language use with their classmates, indicating that a higher proportion of minority language students in a classroom provides additional opportunities for acquiring or maintaining higher levels of proficiency in the minority language. Overall, the ethnic classroom composition does not appear to substantially reinforce existing inequalities between minority and majority students.

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This article describes new immigrants’ levels of destination-language proficiency shortly after taking up residence in Germany. The focus lies on a comparison of refugees from Syria with new arrivals from Italy, Poland, and Turkey, who came as economic immigrants, for family reasons, or as students. The theoretical account builds upon a well-established model of language acquisition, according to which language fluency is a function of exposure, efficiency, and incentives. The empirical study is based on data from the first wave of the ENTRA project (“Recent Immigration Processes and Early Integration Trajectories in Germany”) that covers about 4,600 young adults. The analyses reveal that most individuals improve their proficiency over time. Syrians’ experience a faster learning curve than those of other immigrant groups. The conditions identified as relevant to language fluency largely reflect the findings of previous studies. They indicate that language learning is a general process that, for the most part, does not differ across the four groups. Exposure is the major force driving language acquisition. There are also indications that certain kinds of exposure, such as attending language classes, are especially beneficial for individuals with lower resource endowments. In addition, Syrian refugees profit more than other new arrivals from increased levels of language exposure, such as from taking language courses, pursuing education or being active on the labor market.