A global investor confidence index maintained by State Street Global Markets has dropped in September.

As investors in North America and Asia shied away from risk, the global index fell from August’s revised reading of 91.0 to 86.9, said State Street Global Markets, the investment research and trading arm of State Street Corp., a Boston-based financial services company.

The index uses a scale in which 100 represents a neutral reading.

The State Street Investor Confidence Index was developed by Harvard University professor Kenneth Froot and Paul O’Connell of State Street Associates.

In a statement, Froot commented on the index’s September reading.

“This month’s decline in global investor confidence is the result of diminished risk appetite among both North American and Asian investors,” Froot said. “While diminished growth expectations for Asia account for some of the reticence in that region, the third consecutive decline in confidence among North American investors is more puzzling, especially given recent announcements by monetary policy makers on both sides of the Atlantic. It is clear that in their actual portfolios, institutional investors continue to exhibit caution given the global growth backdrop.”