WASHINGTON — When her oldest child was in kindergarten, Laura Haggerty-Lacalle sat down with her every day to review reading or math, intent on providing that most precious commodity of all: parent time. “Oh my God, it’s the most important thing you can do,” she said.

But when her second child hit the same age, life was more hectic. Now, with a third child, Haggerty-Lacalle, 37, feels good when she gets five minutes to stack blocks or build Legos in her Oak Hill, Va., home. “When you have three kids,” she says, “you’re just trying to survive.”

Within this familiar progression of family life, new research has confirmed what some parents recognize and others quietly fear: Their firstborn children get more of their time than others in the family — on average, 3,000 extra “quality” hours from ages 4 to 13, when sisters and brothers are in the picture.

That’s 25 extra minutes a day with mothers on average and 20 extra minutes a day with fathers across a nine-year span of childhood, according to a study by economist Joseph Price of Brigham Young University.

Some parents find themselves surprised by the lopsided time log, but the big question, experts say, is whether this difference helps explain findings that show firstborn children get better test scores, more education and higher-paying jobs.

“I certainly think it advances our understanding,” said Sandra Black, an economist at the University of California at Los Angeles, who has studied achievement and birth order. Although the reasons for firstborns’ success have not been fully explored, she said, the new study provides one plausible explanation.

Based on federal data from more than 15,000 children whose days were detailed as part of the American Time Use Survey, the new study defined quality time with parents as minutes spent together on such activities as homework, meals, reading, playtime, sports, teaching, arts, religion and conversation. In all categories, firstborns got more, according to the study, published in the Journal of Human Resources.

This was not because of any lack of fair-mindedness, Price said, but rather because of an underlying fact of family life: Parents generally spend equal time with their children on any given day, but they spend less time with their children as the family ages. For example, mothers in two-child families spend 136 minutes a day with their firstborns at age 7. But by the time the secondborn reaches that age, they spend 114 minutes.

These daily differences become a wide gap as the years pass.

Parents often do not recognize the imbalance, Price said, because day-to-day they are fairly equal about their time. “On any given day, you’re more likely to spend a little more time with the second child,” he said. “But it’s still not nearly as much time as you spent with the firstborn when he was that same age.”

Many parents said the time gap was not true for their families. To others, the findings fall in line with the rhythms of family life. Their firstborns led the way in family choices about schools, sports, music lessons and family rules. Every milestone was new.

“The first one has the most profound impact on the parents because you don’t have a clue what you’re doing,” said Dia Michels, 49, a mother of three in Washington, D.C., who recalled that her eldest daughter’s gymnastics classes once set the schedule for the whole family. Younger siblings went along for the ride, and dinners were rearranged.

In Manassas, Va., Kristen Kiefer, 34, a mother of two, said she recognizes that her firstborn, Madeline, 5, “is driving the bus right now about where we’re going and when,” with soccer, play dates and birthday parties. Still, Kiefer said, she is deliberate about making time for her 20-month-old son, Aidan.

Recently, this came up as Kiefer planned a trip for the family. Her son adores Elmo, but her daughter says she has outgrown the “Sesame Street” character. “I decided that, like it or not, we’re taking the baby to Sesame Place while he still enjoys it,” she said.

Sometimes, she said, she wonders whether her secondborn gets short shrift. In the end, she said, “the reality is, with two working parents and two kids … you just never feel like there’s enough time.”

How much the eldest benefits from being a family’s first might not be as certain as experts assume, said Sally MacKenzie, 53, a Rockville mother of four sons who sent her youngest to college in the fall. MacKenzie recalled the words of a relative, who once told her: “Firstborns are the ones you practice on, and it shows.” As a group, she said, firstborns might be more successful by some measures, but “I don’t know if that means they’re happier or more neurotic.”

Despite the time gap, later-born children have advantages, too, Price said. On average, they are raised when families have higher incomes and larger homes; more attend private schools. “The secondborn gets to experience a better life in terms of money, but the firstborn gets more time,” Price said.