WASHINGTON (MarketWatch) – The U.S. economy has been showing green shoots for four years now, but it still isn’t strong enough to stand on its own.

Why is growth so slow? There are a lot of theories about why the economy remains in the doldrums — deleveraging, inadequate demand, rapacious banks, overreaching government, an aggressive Federal Reserve.

Rather than debate these points on a lofty theoretical plane, I thought I’d just compare this recovery to previous expansions in the simple terms of national income accounting (also known as gross domestic product).

My conclusion? Compared with past recoveries, the U.S. economy suffers from low wage growth, premature fiscal contraction and the hangover from the housing bubble.

The basic math of GDP (wonky, but worth it)

Those who took Econ 101 recall that GDP is the sum of all goods and services produced in the United States. In practical terms, this means adding up all of the spending by consumers, the investments in businesses and in housing, and the government spending and investments. Then you net out imports and exports to make sure you only count goods and services produced in the U.S.

Here’s the formula: GDP = C + I + G + X (GDP equals consumption plus investment plus government plus net exports).

In the four years since the recession officially ended, real GDP (inflation-adjusted) has risen at an annual rate of 2.2%, compared with an average growth rate of 3.9% in the four years following the three previous recessions, which began in 1981, 1990 and 2001.

That’s a growth shortfall of about 1.7 percentage points, which largely explains why the unemployment rate is still so high. It takes a higher-than-average growth rate to create all of the extra jobs needed to bring millions of unemployed people back to work.

To explain why the economy is growing so much slower this time, we need to know which sectors aren’t pulling their weight.

In each quarter’s GDP report, the Bureau of Economic Analysis publishes a table showing the contribution of each sector to GDP. (See Table 2 in the GDP report.) In the second quarter, for example, consumer spending contributed 1.2 percentage points to growth, business investment (including inventory stockpiling) provided 1.1 points, residential investment added 0.4 points, government subtracted 0.2 points and net exports contributed nothing. That adds up to 2.5%, which is how much GDP grew at an annual rate.

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In this recovery — over the past four years — consumption has contributed 1.5 points to GDP compared with an average of 2.5 points in the three previous recoveries. Business investment contributed 1 point, equal to the average of other recoveries. Housing investment contributed 0.2 points, compared with an average of 0.5. Government spending subtracted 0.3 points, compared with a typical contribution of 0.5 points. Net exports subtracted 0.1 points instead of the typical 0.5 point subtraction.

To put this in English, the shortfall in GDP is largely due to three factors: Consumer spending is slower than usual, government spending is subtracting from growth instead of adding to it, and housing investments are only about half as strong as usual.

Of the other two sectors, business investment has been close to its historic average, while the improvement in the trade balance is pushing GDP higher.

Explaining the shortfall in growth

The smallest shortfall is in housing, and explaining it is easy: We built far too many homes in the 2000s, and we’re still working down that excess supply of vacant housing units before building many more. Demand for housing is also low, because of high unemployment, because millions of people can’t qualify for a mortgage, and because millions more can’t trade up because they are underwater in their current mortgage.

Explaining the shortfall in government spending is also easy: Republicans insisted that the federal deficit — not unemployment — was the most pressing problem. The White House and congressional Democrats largely went along with the emphasis on reducing deficits. State and local governments sharply cut back their spending and investments in order to maintain required balanced budgets.

The pivot to deficit reduction is not only subtracting from GDP directly, it’s also holding back income growth (through higher taxes and smaller government paychecks and transfers) and is therefore restraining consumer spending as well.

That brings us to the explanation for weak consumption: Mediocre income growth.

Typically, per capita real disposable incomes have grown at an annual rate of 2.4%, but this time per capita real disposable incomes are growing just 0.5%.

Surprisingly, given the rapid growth in the number of retirees collecting Social Security and Medicare, and the large number on food stamps, annual real growth in government transfer payments during this recovery is the slowest in more than 30 years — just 0.7% compared with an average of 3.2%, in part because payments for unemployment insurance have plunged as part of the government’s belt-tightening.

Wages and salaries are also growing very slowly. After adjusting for inflation, total income from wages is up at a 1.1% annual rate in this recovery, compared with an average of 2.8%.

It’s all about wages

Wage growth and economic growth go hand-in-hand. You can’t have strong wage growth without a strong economy creating more and better-paying jobs. On the other hand, you can’t have a strong economy without having strong wage growth: it propels the consumer spending that is the main driver of our economy.

The high unemployment rate explains part of the low wages. The weak labor market weakens workers’ power to bargain for higher pay.

But we also know that wages have been growing slowly for nearly 30 years. It’s not just a cyclical problem; something fundamental has changed in the economy, boosting inequality and rewarding a larger and larger share of national income to the owners of capital rather than to the workers. Profits are at record levels, while wages as a share of national income are at a record low.

Median real wages have been stagnant or falling for more than a decade for the vast majority of workers, according to a new analysis by Larry Mishel and Heidi Shierholz of the Economic Policy Institute.

What can be done

Time will heal some of what’s ailing us. But we can speed up the healing by targeting the underperforming sectors.

1) Assist the recovery in the housing market without creating a new bubble. Clear the market of excess housing. Help more homeowners refinance or stay in their homes. Encourage more lending to borrowers with the ability to repay.

2) Reverse the deficit reduction, temporarily. Raise the debt ceiling. Cancel the sequestration. Pass funding bills for next fiscal year. Get the government on the side of growth. Increase government spending and transfers. Lower taxes, especially for workers who are likely to spend.

3) Increase incomes by giving workers more bargaining power. Raise the minimum wage. Get rid of unnecessary regulations that discourage businesses from hiring. Increase investments in human capital. Stop letting other countries walk all over us in setting the rules of trade.

Most of all, we need to make growth a priority.