The world's largest pension fund aims to boost and broaden its socially responsible assets after buying them for the first time this year.

Hiromichi Mizuno, who helps manage $1.3 trillion in assets for the Government Pension Investment Fund (GPIF) as its chief investment officer, said the fund wants to increase allocation to the point where it impacts results. For that to happen, GPIF President Norihiro Takahashi has said ESG assets should reach around 10 percent of its Japanese equity holdings, Mizuno said. GPIF announced last week that 3 percent of its domestic stock portfolio of ¥35 trillion is in three environmental, social and governance (ESG) indices.

"Just 3 percent doesn't have much effect on our performance, there's no point unless it makes a difference on our performance from a fund manager's perspective," Mizuno said in an interview in Tokyo on Tuesday. "If you think about the holdings having an effect on our performance, 3 percent is just too small."