Mary and John were the most popular baby names in the U.S. for a number of decades. Eventually, they were replaced with names like Robert, James and Lisa. Then Robert, James and Lisa were overthrown by names like Jessica, Jacob and Emily.

Most people already know that the most popular baby names have evolved over the years. What they might not realize, though, is that the pervasiveness of the "most popular" names in the nation has changed over time as well. Simply put, today's top names are not as dominantly popular as the top names of years past.

The following list displays the #1 male and female baby names by decade. More important, though, are the numbers in front of each name -- these indicate what percentage of male and female babies (respectively) born in that decade were given that name.

1880s: 7.6% (John) and 6.5% (Mary)
1890s: 6.6% (John) and 5.6% (Mary)
1900s: 5.8% (John) and 5.2% (Mary)
1910s: 5.4% (John) and 5.6% (Mary)
1920s: 5.1% (Robert) and 5.7% (Mary)
1930s: 5.4% (Robert) and 5.2% (Mary)
1940s: 5.3% (James) and 4.3% (Mary)
1950s: 4.1% (James) and 3.2% (Mary)
1960s: 4.2% (Michael) and 2.6% (Lisa)
1970s: 4.1% (Michael) and 3.5% (Jennifer)
1980s: 3.5% (Michael) and 2.5% (Jessica)
1990s: 2.3% (Michael) and 1.5% (Jessica)
2000-2005: 1.4% (Jacob) and 1.3% (Emily)

As you can see, the early #1 names experienced widespread usage. As the years elapsed, though, the usage of these top names waned. (Being #1 isn't quite what it used to be, I guess.)

And it isn't just the #1 names being used less frequently. Compare the top ten names of 1905 to those of 2005:

Top Ten Names of 1905
1. John (5.6%) and Mary (5.2%)
2. William (4.5%) and Helen (2.2%)
3. James (4.2%) and Margaret (1.8%)
4. George (3.0%) and Anna (1.8%)
5. Charles (2.5%) and Ruth (1.6%)
6. Robert (2.4%) and Elizabeth (1.3%)
7. Joseph (2.3%) and Dorothy (1.3%)
8. Frank (2.0%) and Mildred (1.2%)
9. Edward (1.7%) and Marie (1.2%)
10. Thomas (1.5%) and Alice (1.2%)

Top Ten Names of 2005
1. Jacob (1.2%) and Emily (1.2%)
2. Michael (1.1%) and Emma (1.0%)
3. Joshua (1.1%) and Madison (1.0%)
4. Matthew (1.0%) and Abigail (0.8%)
5. Ethan (1.0%) and Olivia (0.8%)
6. Andrew (1.0%) and Isabella (0.7%)
7. Daniel (0.9%) and Hannah (0.7%)
8. Anthony (0.9%) and Samantha (0.7%)
9. Christopher (0.9%) and Ava (0.7%)
10. Joseph (0.9%) and Ashley (0.7%)

In 1905, about 30% of all male babies were given a top ten name, versus just 10% a century later. For females, the proportion decreases from about 19% to about 8%.

So what does this all mean? Well, as the reigning baby names become relatively less prevalent in society, the race for #1 will become tighter -- and a larger pool of names with a real shot at reaching the top spot will make the "most popular" rankings more volatile and less predictable from year to year.