Differences between paid and volunteer FOSS contributors

There's a lot of debate these days about the impact of the increasing number of paid developers in FOSS communities that started as volunteer efforts and still have significant numbers of volunteers. Evangelia Berdou's PhD thesis "Managing the Bazaar: Commercialization and peripheral participation in mature, community-led Free/Open source software projects" contains a contains a wealth of information and insights about this topic.

Berdou conducted interviews with members of the GNOME and KDE projects. She found that paid developers are often identified with the core developer group which is responsible for key infrastructure and often make a large number of commits. Furthermore, she suggested that the groups may have different priorities: "whereas [paid] developers focus on technical excellence, peripheral contributors are more interested in access and practical use".

Based on these interviews, she formulated the following hypotheses which she subsequently analyzed in more detail:

1. Paid developers are more likely to contribute to critical parts of the code base.
2. Paid developers are more likely to maintain critical parts of the code base.
3. Volunteer contributors are more likely to participate in aspects of the project that are geared towards the end-user.
4. Programmers and peripheral contributors are not likely to participate equally in major community events.

Berdou found all hypotheses to be true for GNOME but only hypothesis two and four were confirmed for KDE.

In the case of GNOME, Berdou found that hired developers contribute to the most critical parts of the project, that they maintained most modules in core areas and that they maintained a larger number modules than volunteers. Two important differences were found in KDE: paid developers attend more conferences and they maintain more modules.

Berdou's research contains a number of important insights:

• Corporate contributions are important because paid developers contribute a lot of changes, and they maintain core modules and code.
• While it's clear that the involvement of paid contributors is influenced by the strategy of their company, Berdou wonders whether another reason why they often contribute to core code is because they "develop their technical skills and their understanding of the code base to a greater extent than volunteers who usually contribute in their free time". It's therefore important that projects provide good documentation and other help so volunteers can get up to speed quickly.
• Since many volunteers cannot afford to attend community events, projects should provide travel funds. This is something I see more and more: for example, Debian funds some developers to attend Debian conference and the Linux Foundation has a grant program to allow developers to attend events.
• Paid developers often maintain modules they are not paid to directly contribute to. A reason for this is that they continue to maintain modules in their spare time when their company tells them to work on other parts of the code.

Personally, I'm also wondering why there are some significant differences between GNOME and KDE. For example, about 67% of contributors where paid to work on GNOME whereas only 38% were paid to work on KDE. Why do some projects attract more commercial involvement whereas other projects continue to be mostly volunteer based? [Update: it was pointed out that I made a mistake with these numbers. In reality, 37% of GNOME developers are paid to work on GNOME or GNOME and other FOSS according to the PhD thesis. The number for KDE was correct (38%). So according to this PhD there is not a lot of difference in the percentage of paid developers between GNOME and KDE. My original question still stands though because there are many projects that don't attract much commercial involvement at all.]