Define “traditional typographers.”

Also, give us some examples of psychology research that you feel might be contributing to type design, and we will be better able to confirm or deny your thesis.

(I have seen fonts designed by scientists, and they suck, but I don’t think that is the point you are aiming at.)

how knowledge of psychology can lead to more effective design

Advertisers have been using knowledge of psychology for years... but again, that might not be what you are talking about.

scientists or psychologists

Aren’t psychologists scientists?

scientists or psychologists working in the field of design.

They may collaborate with designers, but that’s not the same thing. You might just as well say that in such a collaboration, designers would be working in the field of science.

(I have seen fonts designed by scientists, and they suck, but I don’t think that is the point you are aiming at.)

Do you have any examples?

How valid do you think Tinker and Paterson’s research into legibility for example is to typographers and graphic designers? Many typographers say that typography has so many different variables that you can’t narrow it down to saying 10/12pt Bembo is the most legible and readable typeface, for example through testing in a lab?

Tanya, this has been discussed ad infinitum at Typophile.
To find the threads, use Google, rather than Typophile’s search function.
Microsoft’s Kevin Larson (kevlar), a scientist, has contributed to many of them, so you might enter his name in the search field.