A variety of highly analytical studies, mostly in the USA, suggest that people in informal mentoring relationships are much more satisfied with them. Among the reasons suggested for this are:
Informal relationships take longer to get off the ground and tend to last longer overall, so there is more opportunity to create strong trust and to achieve medium-term goals. Formal relationships are often under considerable time pressure. Informal mentors are less likely to be in the role out of some form of obligation; they are there because they want to be. (There is evidence that altruistic mentors are less effective than those who see benefits for themselves in the relationship. ) Many companies with formal schemes put subtle pressure on managers to become mentors as a way to demonstrate their commitment to people development.
Informal mentors tend to have better communication and coaching skills than formal. (This is a matter of numbers - formal schemes often create increased demand that can be filled only by relaxing the competence criteria. In informal mentoring, the people most likely to put themselves forward - toxic mentors excepted - are those who have confidence in their own competence to perform the role. )
Broadly, these studies suggest that informal mentors offer stronger elements of friendship and empathy than formal mentors. Most of the other differences identified relate to the mentor's willingness to act as a sponsor to the mentee - something seen as positive in traditional US mentoring but as a practice to avoid in European developmental mentoring, which places much more emphasis on helping the mentee become more self-resourceful.