As one of six children being raised by her widowed father, 16-year-old Sonora Smart-Dodd felt bad for dad while sitting through a Mother's Day sermon at her Spokane, Washington, church in 1909. Recruiting her reverend and the local YMCA, Smart-Dodd spearheaded an annual celebration of fatherhood, first held on June 19, 1910, just a couple weeks shy of her dad's birthday [source: Scott]. Then, after decades of government and advertising publicity campaigns, Father's Day finally became a national holiday in 1972, when President Richard Nixon -- himself the father of two daughters -- signed a proclamation reserving every third Sunday in June for its observance.

But what exactly does Fathers' Day celebrate about the 70 million men who claim the title of "Dad" in the United States [source: U.S. Census Bureau]? Unlike motherhood, which involves a specific set of biological processes, fathering can seem like a scientifically nebulous concept. In the animal kingdom, in fact, fatherhood is a bit of an anomaly. Certainly, sexual reproduction requires both male and female contributions, but the dad doesn't always have to stick around to ensure offspring survival. Male chimpanzees and bonobos, humans' closest primate relatives, don't monkey around with their progeny at all, for instance, and only 5 percent of mammalian species practice routine paternal caregiving [source: Martin]. In that way, invested fatherhood is a uniquely human trait that anthropologist Margaret Mead once referred to a "biological necessity and a social accident" [source: Hewlett].

Those social trappings and expectations of fatherhood have been in constant flux for centuries. They also vary from society to society, ranging from the East African Kipsigi men -- who don't hold new infants for the first year -- at one end of the spectrum, to the Central African Aga men -- who collectively spend more time with their kids than any other papas on the planet -- on the other [source: Cooney]. Meanwhile, Western fellows tend to fall somewhere in between, spending a handful of hours with their sons and daughters during the week.

Beyond clocking in and out of hours with the little ones, the cultural history of American fatherhood in particular also reveals a deeper relationship between men and parenting that's driven less by family dynamics than economic inertia.