The work, led by Lillian Turner de Tormes Eby, used database searches to gather 173 samples where mentoring had been investigated. All these samples contained data on protégé perceptions of mentoring, allowing the study to amass common findings and arrive at sizes for the different effects. The analysis spanned across academic and working contexts, and the authors found that the findings rarely differed between these contexts, doing so only in degree, not in nature.
First off, greater similarity between protégé and mentor on deep features such as aligned values or attitudes was solidly related to three key measures of mentor value: relationship quality (liking of the mentor and satisfaction with how the relationship has unfolded), psychosocial support (counselling and offering acceptance), and to a lesser extent instrumental support (sponsorship or providing visibility in organisations). Having a similar background and experiences provided a more modest boost to instrumental support and a smaller one with relationship quality. Meanwhile, surface level similarities between mentor and protégé such as race, age and gender, turned out to be in aggregate almost irrelevant, with tiny effect sizes.
In terms of the process of mentoring, protégés in more informal relationships received slightly more support of both types, as evidenced by the small correlations with those variables. And more frequent interactions were helpful in terms of all three key measures, especially for the workplace-based samples. In terms of what the protégé brought to the table, those with more social capital - supportive friends and family - appeared to be better able to form a relationship of higher quality and felt they could gain more usable instrumental assistance.
What did these effects produce? The more the relationship embodied any of the three core components, the more satisfied the protégé, the stronger their sense of affiliation with the organisation, and the less likely they were to plan to leave the organisation. In addition, the two types of support were associated with greater learning. More instrumental support and higher relationship quality were both associated with stronger perceptions of career success. All these effects were between small and medium in size. The analysis looked at health outcomes and found a no effects beyond a small correlation between more psychosocial support and less workplace strain.
To summarise, mentor-protégé relationships are stronger when the two align on deep features and to a lesser extent in terms of common experiences, when the contact is regular and informal, and when the protégé has other relationships to offer them solid foundations. In terms of outcomes, the authors note in concluding that 'for the typical protégé, the benefits of mentoring are likely to be more limited in both scope and magnitude' than have sometimes been touted.