When the latest performance figures for the three years to the end of July were compiled by financial publisher Citywire, 105-year-old Baillie Gifford had six of the top 10 spots. First State held three while Standard Life Investments (SLI) had one.
Mr Brett and Ms Whitley manage the £331.95 million Baillie Gifford Japanese fund, running what is intended to be a concentrated portfolio with relatively low stock turnover.
Currently nearly one-fifth of the portfolio is invested in manufacturing and machinery stocks including a 3.7% holding in Fuji Heavy Industries.
Some 17.5% has been put into information, communication and utilities sector through stocks such as telecommunications operator KDDI, the fund's largest holding at 3.8%.
Around 14.6% of the portfolio is in electricals and electronics.
Such investments allowed Mr Brett and Ms Whitley to hold off the challenge of Jonathan Asante, manager of the First State Global Emerging Markets, First State Global Emerging Markets Leaders, St James's Place Global Emerging Markets and First State Latin America funds.
They deposed him in the previous month after the former Framlington money manager spent four months at the top.
Ms Whitley read experimental psychology at Oxford University.
She joined Baillie Gifford after graduation in 1980 and became a partner in 1986.
She has run the Japanese equities investment team since 2001.
Mr Brett joined Baillie Gifford's graduate scheme in 2003 after earning a PhD in Psychology at Bristol University.
He had previously graduated from Cambridge University. He has worked on the Japanese fund since 2008.
Artemis had a successful month with its stable of Citywire-rated mangers rising from six to eight as Ruth Keattch and Derek Stuart, managers of the UK Special Situations Fund received recognition for improved performance.
This is the second-highest total among funds houses with a strong Scottish presence, behind Baillie Gifford at 15.
Citywire's rankings are based on risk-adjusted performance. Fewer than one-fifth of fund managers in the UK qualify for a Citywire rating, of which there were 310 in the month.