George Washington, the “Father of the Nation,” had no biological children of his own. But during his 40-year marriage to Martha, the Revolutionary War hero and first president presided over a Mount Vernon estate filled with her children and grandchildren, and by their accounts was a beloved father figure.

Why did George and Martha have no children of their own? There’s almost nothing in the historical record that conclusively answers what was then (and now) a private question, but that hasn’t stopped people from guessing. Modern theories range from tuberculosis-induced sterility to, in Martha’s case, a severe bout of measles.

George and Martha were both in their late twenties when they married and fully expected to have children together. In Washington’s day, it was common to blame the woman for fertility issues, but Mary V. Thompson, research historian at Mount Vernon, says that Martha had four children with her first husband, Daniel Parke Custis, and “there’s no evidence that there was a problem.”

If Washington’s lack of biological children bothered him, he left no record of it. Historians point to one letter to his nephew in which a 54-year-old Washington discusses the remote possibility of future heirs. If he were to die before Martha, Washington insists that there’s a “moral certainty” that no illegitimate heirs will come out of the woodwork. And if he were to outlive Martha and remarry, there still wouldn’t be any kids.

“[S]hould I be the longest liver, the matter in my opinion, is hardly less certain,” wrote Washington, “for while I retain the faculty of reasoning, I shall never marry a girl; & it is not probable that I should have children by a woman of an age suitable to my own, should I be disposed to enter into a second marriage.”

But the lack of his own biological children didn’t mean that Washington was childless. Martha’s two oldest children had already died by the time she remarried, but Washington became the legal guardian of her two younger children: four-year-old John Parke Custis (known as Jacky) and two-year-old Martha Parke Custis (known as Patsy).

From his letters, we get a clear picture of Washington as a somewhat stern and formal parent, but also a loving father who only wanted the best for his children and eventually his grandchildren.

“It seems like [Washington] was a good father figure to the kids,” says Kathryn Gehred, research editor with The Washington Papers at the University of Virginia. “He’s always writing letters to Martha’s children and to the grandchildren they take in after both of those children die. He’s always giving people advice—very rarely listened to—but you can tell that he took on a big role.”

Washington placed great importance on education, especially for the male children and grandchildren in his family. Because Washington’s own father died young, he never received a formal education beyond grammar school.

Washington was sorely disappointed when boys in his family seemed to lack interest in school and preferred the relaxed life of country gentlemen. In a letter to Jacky’s schoolmaster, Washington complains that Jacky is returning from a summer break, “His Mind a good deal relaxed from Study, & more than ever turnd to Dogs Horses & Guns.”

Washington asks the schoolmaster to make sure that Jacky doesn’t sneak out and get into trouble, “rambling about at Nights in Company with those, who do not care how debauchd and vicious his Conduct may be.” A worried father, Washington insists that “I have his welbeing much at Heart, & shoud be sorry to see him fall into any vice, or evil course, which there is a possibility of restraining him from.”

Washington’s relationship with his girls was less strained, but also tinged with tragedy. He fell in love with little Patsy and was the only father she ever knew. Sadly, she was plagued by epileptic fits starting in her early teens and died suddenly at 17 with a weeping Washington by her bedside.

“He was very upset,” says Thompson. “Apparently, she had been doing better, and he and Martha were both terribly surprised that it happened and just devastated.”

The day Patsy was buried at Mount Vernon, Washington penned a letter to his brother-in-law relating the sudden loss of his “Sweet Innocent Girl” and its debilitating effect on Martha, which had "almost reduced my poor Wife to the lowest ebb of Misery."

Eight years after Patsy’s death, Washington had a second act as the de facto father of two of his grandchildren. When Jacky died in 1781, George and Martha took in his two youngest children, two-year-old Eleanor Parke Custis Lewis (known as Nelly) and infant George Washington Parke Custis (affectionately called Washy).

When the Marquis de Lafayette visited Mount Vernon in 1784, he wrote of the warm relationship between the towering war hero and his three-year-old grandson. He described “a very little gentleman with a feather in his hat, holding fast to one finger of the good General’s remarkable hand, which (so large that hand!) was all the tiny fellow could manage.”

As Washy grew up, he inherited his father’s distaste for school. In a letter to the president of Princeton, where Washy was about to flunk out, Washington vents his frustration.

"From [Washy’s] infancy, I have discovered an almost unconquerable disposition to indolence in every thing that did not tend to his amusements,” wrote Washington, “and have exhorted him in the most parental and friendly manner, often, to devote his time to more useful pursuits…”

When Washy eventually dropped out of Princeton and came home to Mount Vernon to “study,” Washington wrote him with classic fatherly advice—“Rise early, that by habit it may become familiar, agreeable—healthy—and profitable”—and some good old-fashioned nagging.

“[T]he hours allotted for study, if really applied to it, instead of running up & down stairs, & wasted in conversation with any one who will talk with you, will enable you to make considerable progress in whatsoever line is marked out for you: and that you may do it, is my sincere wish.”

The stoic Washington we know from portraits was surprisingly keen on offering love and marriage advice to his granddaughters and nieces. When his 18-year-old granddaughter, Elizabeth Parke Custis Law, was discouraged that her younger sister had beaten her to the altar, Washington warned her of marrying only for love.

"Love is a mighty pretty thing; but like all other delicious things, it is cloying,” Washington wrote Elizabeth, “and when the first transports of the passion begins to subside, which it assuredly will do, and yield, oftentimes too late, to more sober reflections, it serves to evince, that love is too dainty a food to live upon alone, and ought not to be considered farther than as a necessary ingredient for that matrimonial happiness which results from a combination of causes."

Washington’s fatherly advice was routinely ignored. In a later letter to Elizabeth, Washington warns her about marrying an older man: “[F]or youth and old age, no more than winter & Summer, can be assimilated—the frigidity of the latter, cannot be kept in unison with the warmth of the former: and besides the habits of the two, are widely dissimilar.”

Two months later, Gehred says, Elizabeth became engaged to a man “twice her age.” Fifteen years after their marriage, the union ended in divorce.

Washington never formally adopted any of Martha’s children or grandchildren, but that didn’t make him any less of a father in their eyes. In 1776, a year into the Revolutionary War, a now-married Jacky was moved to write a heartfelt letter to Washington expressing what he had never been able to say in person.

"It pleased the Almighty to deprive me at a very early Period of Life of my Father, but I can not sufficiently adore His Goodness in sending Me so good a Guardian as you Sir,” wrote Jacky. “Few have experience'd such Care and Attention from real Parents as I have done. He best deserves the Name of Father who acts the Part of one."