Heather Jacobs, a chain-­spa masseuse in Simi Valley, Calif., never knows how much money is coming her way. When her spa charges $99 for a 55-­minute massage, she makes $18. But if she books just two massages in a six-­hour workday, the spa must raise her day’s pay to minimum wage, which is $9 an hour. Jacobs, who is 28, can’t be sure how many hours she will get or how many walk-­ins will choose her — a 5-foot-­2, 96-­pound masseuse who looks as if she could manage only a 1 on the four-­point pressure scale, though she is really a 3, verging on a 4. Her paychecks, which come every two weeks, have been as high as $700 and as low as $90. She also freelances for a gym, offering passers-­by free massages on the chance they might join; the gym doesn’t pay her, so for that she earns only tips. She seeks out private clients too, especially around the 27th of the month, when her credit-­card bill, with its $90 minimum payment, is due.

“That’s when I’m desperate,” she said, “going everywhere I can, just to find people to rub.”

Jacobs, in her habitual bright pink lipstick and matching headband, was explaining all this via Skype, from her “little box of a house” in Simi Valley, to Jane Leibrock, 33, who had the more muted look of the fortunate and was sitting in the Oakland headquarters of Even, the start-­up where she works. Leibrock — an alumna of Yale; a onetime Beijing resident and Mandarin speaker; a street-­fashion blogger; a tweeter of first-­world problems (“My personal hell would be having my earbuds pulled out over and over throughout eternity”) — used to study the user experience of privacy settings for Facebook. Now she is a researcher for Even, which proposes, for a fee, to help solve the problem of income volatility.

Leibrock wanted to know everything about Jacobs’s financial life. Jacobs gamely obliged, though at times she became flustered or teary. Thinking about money gives her a jolt, “like you’re about to get into a car accident,” she later told me. There’s the $3,700 for massage-­school tuition that she still owes on her credit card; the $60 a month for drugs for her anxiety and bipolar disorder, which she might skimp on again; the old debt she is paying down for her husband, a student and part-­time delivery driver for a Red’s BBQ. Splurge on Walmart groceries this week or stick to the dollar store? Because bills come like clockwork and income does not, Jacobs never knows.

Income volatility has been called America’s “hidden inequality.” The economists Karen Dynan, Douglas Elmendorf and Daniel Sichel estimated in a Brookings Institution paper that American household incomes became 30 percent more volatile between the early 1970s and the late 2000s, and that in recent years, more than one in 10 American households took in half the annual income that they did the previous year. The Federal Reserve found in 2014 that nearly a third of American households experienced significant income swings. The volatility is hardest, of course, on the poor, who don’t just earn less than the better-­off but also earn their lower incomes more choppily, the money coming in irregular bursts, surging in some weeks, vanishing in others, always making a mockery of plans. Many poor people earn more each year than they spend, but on a given day, they don’t have the cash to handle the expenses due. Payday loans, pawn shops, credit cards, overdraft fees and such fill the vacuum and make things worse, levying a vast toll in interest, fees and stress.