Stocks are starting the week in the red, with the Dow Jones Industrial Average (DJINDICES:^DJI) and the broader S&P 500 (SNPINDEX:^GSPC) down 0.14% and 0.29%, respectively, as of 10 a.m EST.

Today
Between 4 p.m. and 5:30 p.m. EST today, Fed Chairman Ben Bernanke will speak at the University of Michigan's Ford School of Public Policy about monetary policy, recovery from the global financial crisis, and long-term challenges facing the U.S. economy. He will then take questions from the audience and from Twitter via #fordschoolbernanke.

This week
Wednesday: JPMorgan Chase reports earnings. December's Consumer Price Index (i.e., inflation) data will be released.

Thursday: Citigroup and Intel report earnings.

Friday: General Electric reports earnings.

Three sectors -- and one stock -- carrying the S&P 500
The reporting season gets underway in earnest this week, and it will be worth tracking fourth-quarter results closely for a couple of reasons.

First, it may give investors some sense of the effect that the "fiscal cliff" had on consumer and business activity during the period. This isn't just a sunk cost, either; with government spending cuts simply postponed for two months and a political quarrel over the debt ceiling looming, the fiscal cliff is still on our horizon. Whatever impact it had last quarter, it's not unreasonable to expect a similar effect in the current quarter.

Second, the following graph suggests that U.S. corporate profit margins peaked in 2012. The graph shows operating profit margin for the S&P 500 on a trailing-12-month basis. Note that the red bar at right represents the calendar year 2012, based on actual results through the first three quarters and the current estimate for the fourth quarter. That bar assumes that margins start rising again after they had already begun to dip. That's worth at least one raised eyebrow.

Source: S&P Dow Jones Indices.

In fact, if one drills down to the sector level, fourth-quarter estimates put margins significantly below their highs of the past two-and-a-half years for all but three sectors: technology, financials, and consumer discretionary. The greatest -- and most consequent -- outlier in that regard is Apple (NASDAQ:AAPL). Over the 12-month period ending last September, the company's 26.7% net income margin is higher than it has been during any corresponding period in the past 20 years. Apple is the largest weighting in the S&P 500 at 3.7%. Investors had better hope this fruit doesn't bruise.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.