Low interest rates and a U.S. stock market that is still down 2 percent for the year have taken their toll on corporate America’s pension plans. Pension liabilities grew 1 percent in the third quarter, as returns fell 2.5 percent, according to global consulting firm Milliman. The increase in the pension shortfall has been largely driven by investment losses in August and September, low discount rates used to value pension liabilities, and recent studies showing that people are expected to live longer over the next 20 years.