NEW YORK (AP) -- Shares of life insurers fell Friday as an analyst said concerns about investment quality and credit cycle exposure have become a legitimate valuation consideration for life insurers including Genworth Financial Inc. and Prudential Inc.

Insurers make money by collecting premiums from customers they insure, and by investing the money they collect that is not paid out in claims. They commonly invest the money in bonds and other securities.

Life insurers' exposure to residential mortgage-backed securities, with the exception of Prudential, is fairly limited, but risk to the industry would rise if the credit cycle downturn spread to commercial or corporate markets, Citi Investment Research analyst Colin Devine wrote in a note to clients.

A bond guarantor collapse would also likely not pose a threat except for Genworth, he said. Bond insurers make payments to cover principal and interest when issuers are unable to pay their obligations. Turmoil in the credit markets is pressuring bond insurers as prices of bonds and other forms of debt sinks.

"Genworth makes our watch list because of its inherent exposure to (the) housing market via its mortgages insurance business, augmented by its large sub-prime/Alt-A holdings, plus its large holdings relative to peers of municipal bonds, Devine wrote.

Genworth shares fell $1.16, or 5.1 percent, to $21.46 in midday trading and earlier touched a 52-week low of $21.09.

Prudential also fell. At 20.5 percent, its risk asset ratio is the highest among the life insurers, followed by MetLife Inc. at 15.6 percent, Devine said. Prudential shares fell $3.63, or 4.3 percent, to $80.67 in midday trading. Earlier the shares dropped to a year-low of $79.92. MetLife shares fell $1.83, or 3.1 percent, to $56.43 and earlier touch a year-low of $54.77.

Aflac Inc., Principal Financial Group and Unum Group appear to have the highest quality portfolios, each having modest exposure to high-risk assets, structured securities and municipal bonds, he said.

Aflac lost $2.92, or 4.7 percent, to $59.27; Principal lost $3.24, or 5.2 percent, to $59.70 and Unum slipped 60 cents, or 2.8 percent, to $20.54.

Lincoln National Corp. lost $1.26, or 2.4 percent, to 50.32 and earlier traded as low as $47.84, its lowest trade in a year.