Insurance companies generally rely on income from investments to fund reserving requirements for fixed annuities, but the low interest rate environment has caused 87 firms to fail in that effort, according to a report from Weiss Ratings.

Instead, companies have had to use capital and income from other parts of the company, potentially causing a financial strain on carriers, said Melissa Gannon, vice president at Weiss.

"When these contracts were written, no one anticipated the extremely low interest rate environment that exists today. Well-capitalized companies will be able to handle the gap in investment income, but poorly capitalized companies in this position are at risk of default on their annuity contracts."

Among companies whose investment earnings fell short of reserve requirements are Allianz Life, Sun Life Assurance, Hartford Life & Annuity Insurance Co., Manufacturers Life Insurance Co., Nationwide Life & Annuity and Reliastar Life Insurance Co.

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