Los Angeles police investigate a shooting last month. (AP/Nick Ut)
Los Angeles police investigate a shooting last month. (AP/Nick Ut)

 The late-20th-century crime wave has crested and ebbed, writes Eric Eckholm, but where it came from and where it went remains a mystery:

The reasons for the broad drop in crime remain elusive. It has confounded both those from the right who had predicted that waves of young predators would terrorize communities and those on the left who watched crime fall even through ups and downs in poverty and unemployment. ...

Various experts have also linked the fall in violence to the aging of the population, low inflation rates and even the decline in early-childhood lead exposure.

But in the end, none of these factors fully explain a drop that occurred, in tandem, in much of the world.

That's why today's chart of the day, from The New York Times, is something of a Rorschach test. Everyone sees what they want to see in it. New York Police Commissioner William Bratton puts the decline down to the "broken windows" theory of aggressive policing. Advocates of harsh sentencing laws can argue that more criminals were spending more time in jail, and that others, threatened with long terms, gave up their accomplices once apprehended.

As politicians on the right and the left look for alternatives to being tough on crime, there will be plenty of somber warnings about keeping the streets safe, but the evidence is limited.

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What's in Wonkbook: 1) Keystone XL debate will take weeks 2) Opinions, including Porter and Philip Klein on health care 3) D.C. defies Congress on weed, and more

1. Top story: McConnell will allow a battle royale on Keystone XL

The process will involve weeks of controversial votes on energy. "Senators are bracing for a debate over legislation on the Keystone XL pipeline that could take weeks to conclude, setting up an early test of GOP leader Mitch McConnell's pledge to allow 'regular order' in the upper chamber. ... With Republicans eager to pass the Keystone bill and move on to other priorities, Democrats are warning McConnell not to back away from his pledge to allow an open amendment process — no matter how long it takes." Laura Barron-Lopez in The Hill.

Amendments could force lawmakers to take a stand on global warming. "Senate Democrats are trying to force Republicans into an awkward vote on climate change before the GOP can push through a bill to approve the Keystone XL pipeline. ... Early Tuesday in remarks on the floor, McConnell told senators that he would hold an 'open' but not 'open-ended' process, and he later told reporters he would not stand in the way of a symbolic vote on climate science." Elana Schor in Politico.

Sen. Ted Cruz (R-Tex.) will try to lift the ban on crude exports. "Cruz hopes to attach an amendment lifting the ban, which blocks most overseas shipments of U.S. oil, to legislation moving through the Senate to greenlight the Keystone XL oil-sands pipeline. The push allows Cruz to cast himself as a free-market enabler of the American energy boom, which has unleashed a torrent of crude oil onto the domestic market as a result of hydraulic fracturing and horizontal drilling's one-two punch. It also puts him out front on energy priorities—ahead of others in the Senate considering presidential bids, including Sens. Marco Rubio of Florida and Rand Paul of Kentucky, who have so far kept a low profile during the Keystone debate." Clare Foran in National Journal.

2. Top opinions

PHILIP KLEIN: Republicans must agree on how to replace Obamacare. "A freer, more responsive market would, over time, improve consumer choice, increase quality and reduce costs -- just as it does in other sectors. Trouble is, the party's broad consensus breaks down over precisely how much government intervention is necessary to realize those goals. The philosophical differences lead to policy conflicts." Bloomberg View.

PORTER: Being stingy is expensive, especially in health care. "With a new Republican majority in Congress looking to further the cause of low taxes and less spending, it is easy to forget that tightfisted government imposes very real costs. That we can't easily measure them doesn't mean they don't exist." The New York Times.

FRUM: Sen. Elizabeth Warren (D-Mass.) should run for president. "Lead a fight for America's working people? Hillary Clinton wouldn't lead a fight for motherhood and apple pie if motherhood and apple pie were polling below 70 percent." The Atlantic.

SALAM: Rep. Paul Ryan (R-Wis.) isn't running, which is too bad. "He would have been the candidate of ideas and would have pressed his Republican rivals to think seriously about upward mobility and the need to modernize America’s safety net, among other issues conservatives tend to neglect." Slate.

O'BRIEN: In presidential campaigns, it's ultimately up to the Federal Reserve. "Forget about the gaffes, the horserace, and even the personalities. Elections are about the economy, stupid, and the economy is mostly controlled by monetary policy. That's why every big ideological turning point—1896, 1920, 1932, 1980, and maybe 2008—has come after a big monetary shock." The Washington Post.

GALSTON: Democrats won't give President Obama a free hand on a trade deal. "If President Obama really wants the authority, he will have to fight for it, not just ask for it. That would mean going against the grain of his own party, which he has been reluctant to do throughout his presidency." The Wall Street Journal.

SAMUELSON: The debate over dynamic scoring distracts from the real issues. "Republicans and Democrats find it easier to argue over technical questions that, in the end, won't much affect the budget. ... The truth is that both parties will summon forecasts, whether required by dynamic scoring or created on an ad hoc basis, to advance their agendas." The Washington Post.

NOAH SMITH: Cheap oil means a crisis for Texas. The state shouldn't let this one go to waste. "The 1980's taught Texas the hard lesson that a diversified economy is absolutely essential. In the decade and a half after 1986, Texas's economy grew by 118 percent, while the mining industry (which includes oil and gas) grew only 18 percent. Even since the shale boom began, Texas has seen much greater employment growth in sectors like professional services and health care than in extractive industries. ... That doesn't mean Texas couldn't do even more," especially in higher education and urban planning." Bloomberg View.

Maps of the day: The poor and middle class pay far more in state and local taxes than do the rich, as a percentage of their income. Patricia Cohen in The New York Times.

3. In case you missed it

The D.C. government will challenge Congress on marijuana. City officials argue a provision in the spending bill signed by the president last month barring the District from putting a ballot initiative into effect is worded poorly, and that it doesn't apply retroactively to the legalization measure approved by voters in November. Aaron C. Davis in The Washington Post.

Wall Street wages a war of attrition against Dodd-Frank. "In the span of a month, the nation's biggest banks and investment firms have twice won passage of measures to weaken regulations intended to help lessen the risk of another financial crisis, setting their sights on narrow, arcane provisions and greasing their efforts with a surge of lobbying and campaign contributions." Jonathan Weisman and Eric Lipton in The New York Times.

The Obama administration will issue regulations on natural gas leakage this week. "The proposal would include new Environmental Protection Agency requirements to control methane emissions from new or modified drilling equipment anywhere in the country, according current and former U.S. officials briefed on the plans. In addition, the Department of Interior will impose tighter regulations of both new and existing oil and gas facilities located on U.S. lands." Joby Warrick in The Washington Post.