Ms Bain was in London this week as founder member of the Tennessee STAR (Student Teacher Achievement Ratio) group which presented evidence from a 10-year project in 79 primary schools, involving 7,000 children, which shows that smaller classes improve children's learning.
The project assigned class sizes and teachers randomly in a cross-section of neighbourhoods and then tracked the performances of children.
They found the children in classes of 15 did better, particularly in mathematics and reading, than those in classes of 25. Children from poor backgrounds especially benefited. The same children at 13 and 14 years old were found to be still doing better than their peers who had been placed in larger classes.
The National Commission on Education's final report, which will be published next week, has borrowed heavily on this research and is expected to recommend a lowering of class size in the first two primary years to 20 children.
The Commission, however, finds scant political goodwill on this subject. Ministers have denied any correlation between class size and performance and the Office for Standards in Educations is also lukewarm.
A spokesman from the Department for Education said it was "wary about drawing any conclusions from looking at different class sizes in different countries".
Peter Blatchford, of London University's Institute of Education, said: "We need Government-funded research on class sizes. The Tennessee evidence is most exciting because of the randomly-selected groups at different class sizes.
"It would be very difficult to persuade a local authority or governing body here to replicate that method."
Using the evidence as a lobbying tool, the researchers were able to convince the Tennessee state government to fund the reduction in class size.
Ms Bain said: "We persuaded the politicians that they would recoup the money invested by teachers picking up on special needs earlier and by savings on remedial teaching."