Sheila Riddell, of Stirling University's education department, attributed the improved performance to girls' willingness to study.
Professor Riddell, who last year carried out a survey for the Equal Opportunities Commission on gender differences, commented this week: "Teachers feel that boys are less motivated than girls and are likely to spend their recreational time not on homework but on other things. Young males' peer culture is anti-scholastic."
The trend is evident in the early years of secondary but becomes more pronounced in the later stages. Professor Riddell said schools should pay particular attention to encouraging boys to take academic work seriously, although she was pleased girls were doing so well.
Gender differences are still marked in terms of subject choice, however, with twice as many boys taking physics at Standard grade and Higher.
Peter Kimber, the board's depute chief executive, said: "This is a trend that shows itself south of the border as much as here."
Last year's Standard grade examinations underlined the gap. In English, 13 per cent of girls achieved a band 1 and 31 per cent a band 2. Only 7 per cent of boys reached band 1 and 23 per cent band 2.
At Higher, 16 per cent of girls got an A and 22 per cent a B. Only 12 per cent of boys got an A and 20 per cent a B.
In Standard grade maths, girls marginally outperformed boys by around 1 per cent. At Higher, a 1-2 per cent difference was evident. In Standard grade physics, 38 per cent of girls got a band 1 and 33 per cent a band 2. Figures for boys were 27 per cent and 33 per cent.
At Higher, 17 per cent of girls got an A and 26 per cent a B. Sixteen per cent of boys got a A and 22 per cent a B.
More girls than boys took Standard grade and Higher history. Seventeen per cent of girls got a band 1 at Standard grade and 23 per cent a band 2. Figures for boys were 11 per cent and 19 per cent. At Higher, 14 per cent of girls got an A and 26 per cent a B. The figures for boys were 13 per cent and 24 per cent.
Overall there was a 12 per cent increase in the number of candidates gaining awards in eight subjects at Standard grade while 3 per cent more candidates gained passes in three subjects at Higher.