Many social scientists have studied peer effects. Generally, there is a consensus in the literature that a student’s school- and classmates play an important role in the educational process and can help maintain or exacerbate inequality across demographic characteristics. An important goal of the peer effects literature has therefore been to inform strategies for administrative assignment of students to schools and classrooms with the purpose of increasing educational outcomes (Coleman & Hopkins, 1966).

Since peer effects may appear through different mechanisms and in various contexts, empirical studies across different disciplines such as economics, psychology, and sociology have focused on a wide range of peer characteristics and individual outcomes across different ages, including cognitive and non-cognitive skills, educational choices, mental health, and criminal activities. While economic research has typically focused on the causal identification of peer effects by addressing the endogeneity of the peer group formation through randomized or natural experiments, sociological and psychological research has been more occupied with the peer influence process and the mechanisms leading to peer effects on various outcomes (Andrew & Flashman, 2017; Flashman, 2012; Frank et al., 2008; Gamoran, 2001; Hallinan, 1981).

A growing number of studies look at the effect of gender composition on student outcomes. Within the economic literature this is typically done by analyzing (exogenous) variation in the classroom gender composition on students’ academic achievement, choice of college major and dropout probability. Lavy & Schlosser (2011) and Hoxby (2000) find that an increase in the proportion of female students in the classroom is associated with an increase in student achievement of both boys and girls. Zölitz & Feld (2017) investigate the effect of peer gender composition in university on choice of major and conclude that assignment to more female peers affect major choice of both men and women although in opposite directions. Despite a major interest in the effects of gender composition on individual student’s outcomes, most research until now have focused on the x-side of the peer effects equation, i.e. studying gender peer effects from the compositional dimension. However, research also suggest that boys and girls respond differently to peer relations (Crosnoe et al. 2008) and that particularly girls are prone to the characteristics of same-sex peers (Riegle-Crumb et al. 2006).

In this paper, we study gendered peer effects by investigating the impact of the cognitive and noncognitive composition of same-gender peers on students’ cognitive and non-cognitive outcomes. While previous research has typically focused on either the school/classroom or the friendship group as the relevant unit of analysis, we argue that students are affected primarily by their same-gender peers. Unlike recent research which has primarily focused on the effects peers’ cognitive characteristics on students’ cognitive (and to a smaller extend non-cognitive) outcomes, this paper takes a dual focus by analyzing the effects of both cognitive and noncognitive peer composition on cognitive and noncognitive skills of boys and girls.

We apply within-student across-subjects models to data from 35 countries in the TIMSS 2015 and show, first, that female peers affect mainly females and male peers affect mainly males, second, that while peers’ academic self-concept positively affects individual outcomes, peers’ mean achievement and interest in the subject tend to have a negative effect.

We use data from the 2015 Trends in Mathematics and Science Study (TIMSS), which includes information on fourth and eighth grade students’ achievement in both mathematics and science, student background characteristics, subject-specific attitudes as well as teacher characteristics. The TIMSS data is ideal for studying peer effects to the sampling of entire classes. Furthermore, we utilize the fact that the data contains measurements of student achievement and attitudes for both math and science by applying a fixed effects approach using within-student (and –classroom) differences across math and science (Clotfelter, Ladd, & Vigdor, 2010; Dee, 2005; Schwerdt & Wuppermann, 2011). This design effectively removes bias from any subject-invariant factors at the classroom level. Under the assumption that school, classroom and student factors are not subject-specific, this identification strategy allows us to take account of unobserved heterogeneity which could otherwise bias the results. We estimate fixed effects regressions models for both cognitive (achievement) and non-cognitive outcomes (self-concept). Models are run separately for boys and girls, but including both male and female peer variables for both, to control for the entire variation in the peer variables. Countries were excluded based on more than 40 percent of their students being in gender separated classrooms or only having administered the student-questionnaire related to one of the two subjects, rendering the subject fixed effects approach impossible. The exclusion strategy leaves 35 countries participating with fourth grade students and 21 with eighth grade students. The data includes a pool of items measuring the non-cognitive constructs such as self-concept and interest across subjects and grade-levels. We use these variables both at the individual student level and as peer indicators. We used exploratory factor analysis to derive separate constructs. After excluding cross-loading items a clear pattern in factor loadings emerged, revealing a two factor structure of interest and self-concept. Scales exhibit good to excellent internal reliability across the included countries on both Cronbach’s alpha and confirmatory factor analysis fit indices. We constructed peer variables by aggregating the measures of self-concept, interest and achievement scales at the classroom level, separately for males and females excluding the individual student i. For the analysis we standardized all variables within each country and use multiple imputation to handle missing data.

Although previous studies have defined the peer group in different ways such as the school in general or the specific classroom, in this paper we argue that peer effects are gendered so that primarily peers of the same gender affect individual student outcomes. In addition, in contrast to previous studies who have focused on social and cognitive peer characteristics, this paper extends the literature by investigating both cognitive and non-cognitive characteristics of the peer group. We apply within-student across-subjects models to data from 35 countries in the TIMSS to examine the impact of cognitive and non-cognitive peer characteristics on achievement and self-concept of male and female students. Our results show, first, that across most countries in our sample, female peers affect mainly female students and male peers affect mainly male students suggesting that peer effects are gendered. This result applies to both 4th and 8th grade as well as to both our outcomes of analysis, although the results are clearer for achievement compared to self-concept. In some countries, we also find peer effects on the opposite gender; however, typically these effects are much smaller. Second, while peers’ academic self-concept positively affects individual outcomes, peers’ mean achievement and interest in the subject has a negative effect. This suggest that different peer characteristics operate through different channels resulting in ambiguous effects on individual student outcomes. In conclusion, our analyses show that the peer influence process is a complex one in which cognitive and non-cognitive peer characteristics affect individual outcomes in diverse ways. However, despite this complexity, our results show a very consistent pattern of gendered peer effects.

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