for example by catering to the two languages of instruction in the country.<sup>9</sup> Adaptations to language and details of featured characters (e.g. names and ethnicities) could be easily tailored and transferred to other contexts.

Third, our results are not affected by selective school buy-in. Almost all schools in the country were included in the program and were randomly assigned to one of the two treatment groups or the control group, again strengthening external validity. Within each school, there was no bias in the selection of teachers, classrooms, or students. Therefore, any positive effects of the intervention cannot be attributed to students’, teachers’, or even administrators’ pre-intervention interest in the curriculum. In the field of social-psychological interventions, nearly all randomized controlled experiments in school contexts require school-level buy-in, and impacts could thus be specific to those schools who want or choose to participate, and not necessarily representative of all schools in a country.

This paper is organized as follows. In Section 2, we describe our intervention, including the different treatment arms and the key mechanism of behavior change. Section 3 gives an overview of our survey data on socio-emotional skills and administrative data on GPAs from official records in the country. Our empirical model is outlined in Section 4. The impacts of our intervention on socio-emotional skills and GPAs, on average and by different student sub-groups, as well as robustness checks are presented in the Section 5. Section 6 discusses them against findings in the literature, including cost-effectiveness, and then concludes.

# **2. The Intervention**

The objective of our intervention was to cultivate grit among sixth and seventh-grade students in North Macedonia. Past research highlights the long-term benefits of working with this age group: in early adolescence, motivated behaviors have been shown to have long-term effects on outcomes such as high school retention, college enrollment, or workforce earnings (Allensworth and Easton, 2005; Benner and Graham, 2011; Crosnoe, 2011; Heckman et al., 2014). Hence, we examined whether grit can be built in a critical developmental window that has deep, enduring consequences for a student’s future.

Our intervention covered all public schools with Macedonian and Albanian language of instruction schools with sixth and seventh grade classrooms and at least five students in single-level classrooms.<sup>10</sup> This amounts to a total of 352 schools and about 1,780 classrooms in 80 municipalities in the country.<sup>11</sup> The intervention was delivered nationwide, starting in the semester after the 2016 Christmas holidays (February) and ending by the Easter holidays (March to early May, depending on school-specific holidays), i.e. the third quarter of the school year 2015/2016. It consisted of a curriculum of five consecutive lessons, delivered weekly, which were divided into two parts.

The first part of the curriculum taught students the tenets of _deliberate practice_ (Ericsson, 2008; Ericsson et al., 1993), namely to: (1) identify stretch goals, (2) get feedback, (3) concentrate, and (4) repeat until mastery. Didactic slides were interspersed with activity prompts, engaging images, and exercises such as letter-writing

> 9 The two languages are Macedonian and Albanian. According to the Macedonian State Statistical Office, in the school year of the intervention, 65% of students were in Macedonian language classes or schools, 32% in Albanian, and 3% in Turkish.

> 10 Among the children in North Macedonia, 98% attend public education which is free and compulsory.

> 11 The country has 84 municipalities. Four municipalities and 64 schools were excluded from the sample. Eight (12%) of these schools are “special schools” (e.g., art or music schools, schools for children with special needs), 31 (48%) are too small (fewer than five students in either sixth or seventh grade), 20 (32%) are schools that do not have single-level classrooms (e.g. schools that combine different grades in one classroom, especially in remote areas), and five (8%) are Turkish language schools. The 64 excluded schools represent 11% of all classes in each grade (sixth and seventh) and 7% of the students in each grade. All schools are primary schools and use a standard curriculum.

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