For 16 weeks this past spring and summer, Jim McMillan savored the joy of being a full-time father to his new baby, as well as to his 4-year-old son. A senior manager in the financial services audit practice at Ernst & Young’s New Orleans office, he was among the first group of dads to get nearly four months of fully paid paternity leave.

His second son, Charles, was born last January, but Mr. McMillan, 32, postponed the leave until after the accounting firm’s busy tax season had passed.

“It felt really good to bond with Charles and see him start interacting with the world,” he said. He was there to witness the baby’s developmental milestones such as sitting up, tracking objects with his eyes, and — a big one — figuring out that he could close his fist around his pacifier and enjoy it whenever he wanted.

“And I got to introduce them to ‘Star Wars,’” a beaming Mr. McMillan said. “Henry, my older son, could quote most of the movie.”

Ernst & Young increased paternity leave to 16 weeks from six last year after it realized how important the benefit was to employees, especially the millennial generation. Millennials value parental leave more than earlier generations, so much so that 83 percent of American millennials said in Ernst & Young’s global generational survey of 9,700 people that they would be more likely to join a company offering such benefits. What’s more, 38 percent even said they would move from the United States to another country with better leave policies.

In technology, financial services and other industries where competition for talent is especially cutthroat, it’s like an arms race to provide the best parental leave benefits for fathers as well as mothers. American Express expanded parental leave for both mothers and fathers to 20 weeks this year. And last month, IBM announced that it would double its paternity leave to 12 weeks and increase maternity leave to as many as 20 weeks, up from 14 weeks.

Many of these policies are available to gay men and others who become fathers through adoption or surrogacy.

“Men are doubling down on their daddy duties,” said Karyn Twaronite, global diversity and inclusiveness officer at Ernst & Young. “They want to have impact at home as well as at work.” She said that employees who receive paternity leave are “far more engaged and trusting of the organization because they can live a full life.”

Lukas Staniszewski, a senior management consultant at IBM, whose daughter, Mila, was born in March, said the time off was invaluable. “I wanted to be totally in the present and bond with my baby,” he said.

Spouses also benefit from paternity leave. Mr. Staniszewski’s wife, Maria Potoroczyn, said she could focus on making a career change, knowing that Lukas was attending to Mila’s needs. “It was critical that Lukas was home early on because we don’t have much of a support network,” she said. “We’re alone in New York City, and our families are in Europe.”

For Jim McMillan’s wife, Mary Mustaller McMillan, her husband’s leave from Ernst & Young reduced her stress level when she returned to her job as a lawyer representing children involved with the state’s child protection agency. She faced 100 open cases after her own four-month leave and said she could “catch up much easier without worrying about household stuff and getting the children ready for day care before going to court.”

While some large companies are providing more paid leave, the United States remains the only developed country that doesn’t require paid parental leave. But a few states and cities have begun offering paid leave to some workers. California, for example, provides partially paid parental bonding leave for mothers and fathers that workers fund through paycheck deductions.

Last June, the American Civil Liberties Union filed charges with the Equal Employment Opportunity Commission against J.P. Morgan Chase for discrimination because it wouldn’t consider Derek Rotondo, a fraud investigator, to be his newborn child’s primary caregiver.

According to the complaint, the bank told him he could get only two weeks of leave, not the 16 weeks it usually provides to mothers. To qualify, he would have had to show that the mother had returned to work or was medically incapable to care for the baby. But his wife was in good health and couldn’t return to work because she was a teacher on summer break. The bank declined to comment on the complaint.

Typically in such litigation, men were denied paternity leave or felt discouraged from using the benefit, faced retaliation for having taken time off, or objected to the unequal length of leaves for new mothers and fathers.

“We still have a long way to go with paternity leave in the United States,” said Jane Waldfogel, a professor at the Columbia University School of Social Work. “Currently, the typical father takes some leave, but usually only a week or two.”

Outside the United States, more countries are requiring leave for fathers or expanding the length of such mandated time off. Workers on leave are typically paid from a social insurance fund rather than by their employers.

The average length of paternity leave in European Union countries is 12.5 days, ranging from one day in Italy to 64 in Slovenia, according to a 2016 Rand Europe study for the European Commission. When the leave is seven days or less, fathers receive 100 percent of their income in all countries, the researchers report, but the percentage varies across countries when additional time is offered.

“European countries have different values than the U.S.,” said Ellen Galinsky, president of the Families and Work Institute, a research organization. “There’s the sense that parental involvement is a social good that they want to support.” Studies have shown that fathers who take two or more weeks of paternity leave are likely to continue to be more involved in feeding, diapering and other child care duties.

Yet even when paternity leave is available, many fathers don’t take all or even some of it. Many simply can’t afford it. A Boston College study found that nearly all men feel their employers should offer paternity leave, but 86 percent said they wouldn’t use it unless they received at least 70 percent of their salaries.

Some men still fear that a long absence from work will make them look less committed to their jobs and could derail their careers.

“Men can be fired, demoted, turned down for promotions or get terrible assignments if they take paternity leave,” said Cynthia Calvert, senior adviser to the Center for WorkLife Law at the University of California’s Hastings College of the Law.

Often, it takes a corporate culture change to make men more comfortable about paternity leave. “We have been very explicit about the need to remove any stigma associated with paternity leave,” said Ms. Twaronite of Ernst & Young. “We asked some of our influential, high-performing men to share their stories about taking paternity leave and continuing to be successful at work, if not more successful.”

She is pleased with the progress so far. The company increased paternity leave to 16 weeks in July 2016, and in the first six months, 19 percent of fathers took six or more weeks. Since last December, nearly 40 percent have taken six or more weeks.

Mr. McMillan, the New Orleans audit manager, didn’t hesitate to make full use of paternity leave. “I just made sure clients knew who to contact when I was out,” he said. And he says he received only one work call during his leave.