Simon Johnson, former chief economist of the International Monetary Fund, is the Ronald A. Kurtz Professor of Entrepreneurship at the M.I.T. Sloan School of Management and co-author of “White House Burning: The Founding Fathers, Our National Debt, and Why It Matters to You.”

Jacob J. Lew, the president’s nominee for Treasury secretary, and Mary Jo White, the nominee for chairwoman of the Securities and Exchange Commission, are making financial reformers nervous. The issue is not so much their track record, because neither has worked directly on financial-sector policy issues; it is much more about whom they know.

Specifically, how many people do they know and trust outside the financial sector, away from the sphere of influence of the very large banks? More pointedly, when it comes to thinking about financial-sector policy, who exactly is in their inner circle?

Nobody knows a huge number of people, at least not well. In the language of anthropology and biology, the limit to a person’s social network is known as Dunbar’s Number – which is 147.5, although people often round it to 150. Our brains do not support the interactions required by larger social groups.

More precisely, the predicted size for most human groups, based mostly on the characteristics of our brains, is 100.2 to 231.1 people. For details and caveats, look at Robin Dunbar’s 1993 paper, “Co-evolution of neocortical size, group size and language in humans,” published in Behavioral and Brain Sciences (Issue 16, Pages 681-735). Or just think about the number of people you know well and would rely on for advice, particularly with complex and sensitive issues. When you get to know new people, you often lose track of your previous close colleagues or even good friends.

And what applies to ordinary mortals most definitely applies to the people elected or appointed to run the country. Whenever the world gets complicated – for example, because the financial sector has turned nasty – policy makers need trusted sources and established confidants in order to figure out which way is up and what needs to be done.

If most financial experts you know work at, for example, Citigroup, then you are more likely to see the financial world through their eyes. What is good for Citi (and its executives) will, in your mind, become close to what is good for the United States.

One of the most serious concerns about Treasury Secretary Timothy Geithner was that even though he had never worked in a bank, his social network was full of bankers, mostly because of his time at the Federal Reserve Bank of New York and his close connection with Robert Rubin, a former Treasury secretary and then a director of and senior adviser to Citigroup. In this network, many of Mr. Geithner’s deepest financial-sector connections appear to have been with people who were working at Citigroup in 2007-8. (See, for example, a 2009 article by Jo Becker and Gretchen Morgenson.)

This observation lines up remarkably well with the devastating critique of Mr. Geithner in Sheila Bair’s book, “Bull by the Horns.” The main concern of Ms. Bair, former chairwoman of the Federal Deposit Insurance Corporation, is that Mr. Geithner was too close to Citigroup and saw the world as its senior executives did.

As Treasury secretary, Mr. Geithner hired people from Citigroup – particularly people who had worked closely with Mr. Rubin (in government or in the private sector or both). Now Mr. Lew, a Citi alum with a central position in the Rubin network, is on the verge of becoming Treasury secretary. How likely is Mr. Lew to confront the risks created by unstable global megabanks? Does he personally know anyone who is concerned about the damage that Citigroup is likely to do in the future – or even has a critical view of what it has done in the past?

While Ms. White’s reputation as a prosecutor is second to none, as a defense lawyer she represented executives at several of the largest banks and knows many prominent financial-sector executives. Her husband, John W. White, now a corporate lawyer, had a senior role at the S.E.C. when Christopher Cox was its chairman, a time when the S.E.C. was aiding and abetting excessive deregulation at every opportunity; Ms. White will need to step aside in actions against companies her husband has advised. To whom will Ms. White turn when she wants to understand how to make the financial sector safer?

If confirmed, Mr. Lew and Ms. White face formidable policy agendas. They need to demonstrate both an impressive grip of the details of what works in financial sector reform, as well as the ability to ignore a great deal of whining and to resist other pressure from the megabanks.

The most prominent and urgent case-in-point is that regulators need to complete the Volcker Rule. Mandated by the Dodd-Frank financial reform legislation, this rule will limit the risk-taking of very large banks.

The hitch at this point is primarily the S.E.C. All kinds of excuses can be and have been offered. These have no merit. Congress passed Dodd-Frank more than two years ago, and the regulators have issued draft rules and considered all the comments imaginable. The banking side of the equation – the Federal Reserve, the F.D.I.C. and the Office of the Comptroller of the Currency – is on board. The Commodity Futures Trading Commission will not stand in the way. Everyone is waiting for the S.E.C. to pull the trigger.

If Ms. White cannot get the S.E.C. unstuck, the issue will fall to Mr. Lew, who as Treasury secretary is chairman of the Financial Stability Oversight Council. The legislative intent of Dodd-Frank on this point is clear; I’ve confirmed this by asking legislators what they intended. If a single regulator gets hung up on an issue, the oversight council can override that regulator – to prevent the kind of impasses and lacunas that previously created vulnerabilities in the regulatory system.

Even Mr. Geithner, not the world’s most dynamic reformer, used the power of the oversight council to press the S.E.C. forward on changing the rules for money-market funds. (Interestingly, the Fed has long wanted these rules changed – and Mr. Geithner’s social network obviously includes some top Fed officials.)

Is Mr. Lew willing to push the S.E.C. – perhaps by supporting Ms. White in a forceful fashion – on issuing and implementing the Volcker Rule? Hopefully, this will be a central question in both their confirmation hearings (with the Senate Finance Committee for Mr. Lew and the Senate Banking Committee for Ms. White).

Senator Elizabeth Warren, Democrat of Massachusetts, writing recently for Politico, made the most important point: the administration will only get serious about financial reform when it appoints officials with different attitudes – and, I would say, different social networks – from those who were at Treasury and the S.E.C. over the last four years.

“Personnel is policy,” people in Washington often remark. The next round of appointments, including those at the deputy and under secretary level, is very important. At this point, I am not optimistic about who will get these jobs.

Is the second Obama administration hiring people who understand and can implement financial reform? Or is it again merely promoting people whose social networks are disproportionately tilted toward the big Wall Street banks?