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.

"I think if you told parents that they spend more time with their firstborn, some might say, 'OK,' but many would be shocked because there is this feeling that you treat your children equally," said sociologist Suzanne Bianchi of the University of Maryland.

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 furry red 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. "I go to bed at night thinking, 'I didn't do enough of this for this kid or enough of that for that kid.' " In the end, she said, "the reality is, with two working parents and two kids ... you just never feel like there's enough time."

In Bowie, Md., Damon Kyler, 42, a father of three, noted that the time gap might happen in part because younger children in the family often seek out their older siblings, not their parents. But 3,000 more hours to the firstborn?

"It did surprise me that it was so drastic," he said.

MEASURING QUALITY TIME

During his research, Price said, he discovered the firstborn time gap almost by accident, as he was poring over data on parent-child time, focused on a different topic. "The results were completely surprising and caused a lot of reflection for me as a parent," he said.

In two-child families, firstborn children got about 30 percent more quality time from their parents. Birth-order differences were largest in activities Price thought to be most important, such as reading and playing together. Secondborns prevailed in one category: watching television with parents. Price did not count this as quality time.

Why parents spend less time with children as a family ages was not studied, but Price offered some reasons, including fatigue, age and a waning novelty. In his family, he recalled, the firstborn had an elaborate scrapbook right away, but the scrapbook for his fourth child, 14 months old, has not been started.

Price also pointed out that parents are more involved in driving their children to activities as the firstborn gets older, and driving was not counted as quality time.

MORE TIME NOT ALWAYS BETTER

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."

Lauren Alexander, 38, a mother of two in Cheverly, also noted fairness is not always measured by the numbers. "You can have fair treatment across all of your children, and it may not come in the form of equal time," she said, comparing it to husbands and wives who might not share each chore equally, "but in the end, it's probably fair."

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.

In Oak Hill, Haggerty-Lacalle said that her family is dedicated to reading time before bed and that her husband, Joaquin, takes the older two children on hikes every weekend.

Lately, she has tried to involve her youngest, 2, in household must-dos, such as laundry. It might not be the kind of attention she once gave her firstborn, but her youngest enjoys the task.

"It's not just quality time," Haggerty-Lacalle said. "It's quantity."