We used official registry data to obtain the addresses of families (with children aged from seven to nine) living in the German cities of Bonn and Cologne. Families were contacted via postal mail and informed about the possibility to take part in the mentoring program and the interviews. We informed parents that participation in the mentoring program was not guaranteed due to limited capacity. The interested families were asked to fill out and return a short questionnaire concerning the socioeconomic characteristics of the household and to sign a non-binding letter of intent to take part in the interviews and the mentoring program. We received 1,626 complete responses and, based on the questionnaire, we categorized respondents as either high or low-SES households. SES reflects the level of resources available at the household level, i.e., material, educational, and time resources. Accordingly, a household was classified as low SES if at least one of the three following criteria was met: Low income: Equivalence income of the household is lower than 1,065 Euro. This corresponds to the 30\% quantile of the German income distribution. (ii) Low education: Neither the mother nor the father of the child has a school-leaving degree qualifying for university studies. (iii) Single-parent status: A parent is classified as a single parent if he/she is not living together with a partner.
All low-SES families that expressed interest were invited to take part in the study. To take part, families had to participate in a baseline wave of interviews (fall 2011) and provide written consent to allow the transmission of their addresses to the mentoring program. Importantly, the mentoring program could only accommodate 212 families; hence, out of 590 low-SES families who participated in the baseline wave and gave consent, 212 were randomly selected and constitute our treatment group (Treatment Low SES). The remaining 378 families form the control group (Control Low SES). Randomization was stratified by city (Cologne or Bonn) and SES criteria, for a total of 14 strata. Given the larger relative supply of mentors in Bonn, we assigned a higher share of children in Bonn to the treatment group. Thus, the assignment into treatment was random conditional on location. Therefore, we condition on location for the analyses.
We also invited 150 randomly-chosen high-SES families to take part in the study (not the mentoring program). 122 took part in the baseline wave of interviews and serve as an additional benchmark group (Control High SES).
After the one-year mentoring program, all families that participated in the baseline wave (Treatment Low SES, Control Low SES, and Control High SES) were invited to take part in the post-treatment interviews and experiments (post-treatment wave) in which all of our main outcome variables were elicited.
We exogenously enhanced the social environment of the treated low-SES families with the help of an existing and well-established non-profit mentoring program in Germany, ``Balu und Du''. In this program, elementary school children are provided with a mentor for up to one year. The mentors are predominantly university students (aged from 18 to 30) who volunteer to serve as a mentor for a child. The mentoring program is not targeted toward specific learning goals (such as improved school grades), but rather to enriching children’s everyday lives. A key component of the program is to introduce children to new activities, enable new experiences, and provide feedback; possibly exactly the inputs that are needed for them to develop an accurate sense of their abilities and that might be missing in low-SES families. In practical terms, a mentor typically spends one afternoon per week in one-to-one interactions with his/her mentee. During this time, they engage in joint activities such as cooking, sports, handicraft work, or visiting a zoo, museum, or playground.