1) The process works better in first-person than in third-person voice. Reading "I" seems to forge a stronger connection between the reader's mind and the character's experiences.
2) It also works better when the character shares common ground with the reader, such as coming from the same place. Presumably that laps over into other major identity facets such as sex, gender identity, sexual orientation, ethnicity, religion, profession, age, and so forth. Notice that some of those already define pools of literature; and that many consumers express a desire for characters who are "like them" across multiple forms of entertainment.
These are reasons why I write some of the stuff I do, the way I do. I put ideas in people's heads. I put boots on the ground in some very strange places. I fill in gaps. I look for situations that not only show something interesting, but ideally, contain little nuggets of wisdom or warning that folks can keep for future use. Characters make good decisions and bad ones, practice systems that work and ones that don't, and you get to watch all of that. Somewhere in the stuff I've written, there is probably at least one character who is "like you" -- and if there isn't, you can ask me for one. If you want characters who aren't like you, the diversity is high enough to meet that too.
Just a quick skim through some of my poetic series ...
The Clockwork War -- species survival, mechanical problem-solving, handicapped heroes, determination
Fiorenza the Wisewoman -- life in a rural village, intuitive problem-solving, tolerance for annoying but useful neighbors
Hart's Farm -- people skills in intentional community, healthy relationships in all flavors of sex and romance, tolerance of pretty much anything that does no harm
Kung Fu Robots -- discovery of self and talents, the balance between violence and nonviolence
Monster House -- life in a shared household of very mixed people, family skills, cultural differences, coping with unsympathetic outsiders, being 'other'
The Ocracies -- diverse political systems and worldviews, philosophical problem-solving, diplomacy
The Origami Mage -- rivalry and reconciliation, magical problem-solving, enlightenment
Path of the Paladins -- devotion, service, faith, practicality, gender, nonsexual relationships, spiritual problem-solving, trauma and recovery, never giving up
The Steamsmith -- ethnicity, genderqueer, class, scientific problem-solving, social evolution, cultural differences, bending rules, dealing with overt and covert prejudices
If you look at those, you can see some common threads: different versions of interpersonal skills, multiple flavors of community, assorted sex and gender dynamics, blended or crossed cultures, and all kinds of problem-solving approaches. I like focusing on a specific time and place. I like mixing unexpected characters or situations. Because life will throw all kinds of shit at you, and it helps to see some of the ways that people can deal with that, well or poorly.
All of these storylines have themes. There are big ideas in there. Some of them have a clearly defined plot while others don't. Sometimes a specific plot is implied at the beginning, other times it develops in progress, occasionally one never does; and that's all fine. Some of these series have a moral, or morals; but even those with a very strong one don't put that at top visibility, it's hidden like the core of an apple. These are almost all about people. You folks tend to latch on to characters. I've only got one series that's primarily about place (The Ocracies) and one that's primarily about idea (Lacquerwork). Most are the story of one particular person, or a group of people, told in pretty intimate terms whether first-person or third-person. I write to pull readers into a world, most of the time; and often what pulls me there in the first place is a person. So that's how it goes.
I did notice that, while it's not rare for me to write first-person poems, almost all of the series are written in third-person. I think Monster House is the only one that's consistently first-person. That's interesting. Given the study's suggestion that first-person increases influence, I'm inclined to try more of that.
I do wonder, though, whether the third-person sample was personal or impersonal -- it's possible to write very intimately in third person and really sink readers into a character's head that way; or to write quite impersonally and as objectively as is possible. I suspect that intimate third-person would have more influence than objective third-person. I also wonder if the similarity factor gets stronger or weaker based on how often it triggers. That is, a reader who sees many reflections of self in characters everywhere might well care less about any given one, contrasted with a reader who rarely sees such a reflection and thus cherishes each one with deeper thought. Oh, and they seem to have skipped second-person altogether, which is rare, but even more intimate than first-person. Botch. More studies would be intriguing.
Looping back to my idea of trying more first-person writing: Do you have a preference between first-person and third-person, or do you like both? Do you have a preference between characters very similar to you and very different from you, or do you like both? What kinds of things make you relate to a character more or less? Do you feel that you learn things from reading, or is it just idle entertainment for you?