There really isn't an app for everything. At least not for everyone.

Mr Avi Karnani first noticed that tech gap when he was just out of business school, working on mergers and acquisitions for Citigroup Global Markets. "At my corporation there was all this great financial tech available to help them manage their risk and achieve anything they wanted," he said. "But my friends had high-interest credit cards, and high-fee checking accounts and their student loans were incomprehensible. They were calling me up for help, and they didn't have access to anything like that."

If you run a business, or you're a consumer of means, there's a technology solution for every problem you could imagine, and many you probably can't. But if you're in financial trouble, or have problems typical of low-income people, sorry. It's an analogue world for you.

Still, there are some successes.

Propel's Fresh EBT (Electronic Benefits Transfer) app helps people manage their Supplemental Nutrition Assistance Programme (Snap) benefits. Mr Karnani is the chief executive and co-founder of Alice, a company that automates pre-tax benefits to enable companies to offer them and let more employees use them. Those benefits can add hundreds or thousands of dollars to a worker's pay cheque. But while white-collar employees can usually get them, most hourly workers can't.

Good Call links people arrested in New York City to a lawyer - within seconds. JustFix.nyc helps New York City tenants respond to eviction notices, compel landlords to make repairs, and represent themselves in housing court.

Those four companies got their start at a tech incubator called Blue Ridge Labs, which is part of Robin Hood, a New York City anti-poverty organisation. It's the most productive (and possibly the only) early-stage incubator for tech companies trying to solve a social problem.

Profit-focused tech companies have lots of tech start-up havens where they can get capital, mentorship, colleagues and a chance to show their products; Y Combinator is the best known.

For beginning start-ups seeking social impact, the most prominent organisation has been Blue Ridge.

Companies in a slightly more advanced stage can apply to Fast Forward in the Bay Area of California, and JPMorgan Chase's Financial Solutions Lab has fellowships for work on financial apps. Some companies win fellowships at more than one lab.

In addition, some other big organisations are now making apps. Code for America makes Clear My Record, which automates legal expungements, and GetCalFresh, which signs people up for California's Snap food assistance programme. The International Rescue Committee runs the Airbel Centre, which makes technology to help refugees. (Y Combinator accepts a few social-impact-focused tech companies each year.)

For profit-focused tech companies, accelerators are very helpful. But for social tech, they're a necessity. This technology is generally more complex to create, and much more difficult to finance.

Most low-income people don't have a computer. So apps need to work on a phone. They should be easy to use for people with little financial sophistication. And they need to work with skimpy data plans. "Text messaging is great," Mr Karnani said.

When a business contracts with Alice, its employees link their credit and debit cards to Alice's software. It automatically identifies eligible purchases - healthcare, childcare, commuting expenses - and moves the expense to the pre-tax category.

On payday, Alice texts the employee that the tax she would have paid has been added to her pay cheque. "The phone is going to ding and you see that money," Mr Karnani said. "You bought a Metro card; here's fifty dollars."

That's important, he said. Many poor people have had bad experiences with traditional power structures, such as banks, and don't trust them. "They're not going to do it unless they see someone else they know and trust doing it without becoming a victim," he said. "So it's great if someone can show their phone message to a friend. The friend says, 'Hey, I bought a Metro card and I didn't get fifty dollars. I want my fifty dollars.' "

Those are small challenges for app makers. Here's a very big one: Your audience has very limited ability to pay. So how will you make money?

Financial apps can often figure out a business model; Alice, for example, takes half of the amount of money it saves employers on payroll taxes. But tech companies like JustFix and Good Call are non-profits that must raise money from philanthropy. "It was an uphill battle for us initially, as traditional philanthropic sources of capital are not used to funding technology," said Mr Georges Clement, co-founder and president of JustFix. Fund raising is a constant preoccupation for both companies.

Most venture capitalists won't look at investing in tech firms that have no chance of bringing in big returns - 30 times the value of their investment, or more. To choose those companies, they usually look to what's worked in the past.

Venture capitalist Ben Horowitz of Andreessen Horowitz famously said companies that succeed are founded by "college dropouts with insane ideas going after tiny markets with no way to monetise". (Just to be clear, "college dropouts" in this case means the likes of Mr Mark Zuckerberg and Mr Steve Jobs, not students from the local community college.)

Looking for what has worked in the past is called pattern-matching. It's an obvious strategy, but it's really harmful. "Firms are starting to understand that the historical data is incredibly biased, and pattern-matching just ends up confirming those biases moving forward," Mr Clement said.

Investors apply pattern-matching to both people and products; this hurts socially focused tech. Ms Hannah Calhoon, managing director of Blue Ridge, said: "It's people who don't look like their mental images of founders, and markets that don't look like what people think are markets."

Co-executive director of Good Call, Mr Jelani Anglin, is unusual in the tech world. He's from Far Rockaway, Queens - "median income, $38,000 per year", he said - and his background is in community organising. "Understanding what our community truly needs has been hard for me to portray," he said.

It has been especially difficult to explain to potential investors a feature of Good Call that allows people to list contact information for their friends and family members, so that Good Call can notify them in case of an arrest. (The police take your phone when you're arrested. Got those numbers in your head? Me neither.) "Walking to the store, you can be snatched up. That's a real thing," Mr Anglin said, but it's hard for outsiders to understand.

No venture capital? Then founders have to bring their own money. Few can. "Hey, why don't you leave your job at Google and pay yourself US$40,000 (S$54,300) a year for something that might fail?" said Ms Calhoon. "That's a really different conversation for many people than for someone like Mark Zuckerberg."

This matters because people tend to build products for themselves. They fill a need they know, in a world they know. Mr Anglin cares about making the criminal justice system fairer. Many tech founders might not even have been aware of its inequities.

Tech accelerators can help by providing money, talented peers and mentoring when the market will not. But perhaps the most important thing Blue Ridge provides is something completely different: connections to the community.

Tech is a macho world. The tech hero disrupts existing systems. He goes it alone. That can work with certain commercial tech projects. A bring-me-booze-right-now app that puts together buyers and sellers can be created largely through technology.

But social tech is different. It doesn't replace existing systems. Instead, it makes them work better. "You can't move fast and break things," said Mr Bill Cromie, director of tech innovation at Blue Ridge. "If you are trying to make a difference in serious systemic challenges facing low-income populations, you can't operate in a vacuum."

Mr Clement said JustFix's partnerships with community organisations with a long history of helping tenants are "the thing that has been most important in our organisation's success".

"It really does take a village," Ms Calhoon said. "You are working with others with the exact same goal. Disrupting is disruptive not only to the ecosystem, but also to your own ability to achieve your aim."

Blue Ridge requires companies to include a subject matter expert - one of Mr Anglin's jobs for Good Call. It also ensures that companies talk to community members all the time. It has a Design Insight Group, made up of people in the target audience paid to meet with companies and talk about their lives. And companies go into the community to watch and listen. "We don't design with the community in mind," Mr Anglin said. "We design with the community."

The founders of JustFix (one of whom was active in a tenant's organisation) spent weeks talking to tenants, their advocates, Legal Aid lawyers and housing court judges. "It was without a doubt the most valuable thing that Blue Ridge does," Mr Clement said.

The group came in focused on helping tenants, and the process showed them how.

"Our biggest insight came from observing housing court proceedings," Mr Clement said. "We were sitting in the back for days and days, watching as countless tenants came in and didn't have legal representation - and the landlord did. The tenants would come in with a plastic grocery bag filled with different documents. The judge would ask them to describe needed repairs, and they'd take their phones and start swiping through photos. The judge would say, 'We can't enter this information as evidence, because we'd have to keep your phone.' "

The need to listen to the audience doesn't go away. Mr Malik Reeves, who is the community engagement coordinator for Good Call, was a user of the hotline. "I got a lawyer within 39 seconds," he said. He and his interns spend their time in the community. "The proximity thing is important," he said.

"The ground team is so important," Mr Anglin said. "Sometimes tech is guilty of being analytically smart, but socially inept."

NYTIMES

• Tina Rosenberg won a Pulitzer Prize for her book, The Haunted Land: Facing Europe's Ghosts After Communism. She is a former editorial writer for The Times and the author, most recently, of Join The Club: How Peer Pressure Can Transform The World and the World War II spy story e-book D For Deception.