The nation's largest youth football program, Pop Warner, saw participation drop 9.5 percent between 2010-12, a sign that the concussion crisis that began in the NFL is having a dramatic impact at the lowest rungs of the sport.

According to data provided to "Outside the Lines," Pop Warner lost 23,612 players, thought to be the largest two-year decline since the organization began keeping statistics decades ago. Consistent annual growth led to a record 248,899 players participating in Pop Warner in 2010; that figure fell to 225,287 by the 2012 season.

Pop Warner officials said they believe several factors played a role in the decline, including the trend of youngsters focusing on one sport. But the organization's chief medical officer, Dr. Julian Bailes, cited concerns about head injuries as "the No. 1 cause."

"Unless we deal with these truths, we're not going to get past the dropping popularity of the sport and people dropping out of the sport," said Bailes, a former Pittsburgh Steelers neurosurgeon whose 10-year-old son, Clint, plays Pop Warner outside Chicago. "We need to get it right."

The statistics, which have not been previously disclosed, are consistent with declining participation rates reported in youth football across the country. USA Football, a national governing body partially funded by the NFL, said participation among players ages 6 to 14 fell from 3 million to 2.8 million in 2011, a 6.7 percent decline.

Pop Warner, founded in 1929, is the largest youth football organization in the world; the NFL players union estimates that 60 to 70 percent of all NFL players started in the program. After years of steady growth, the organization saw participation drop 5.7 percent for the 2011 season, according to the internal Pop Warner data provided to "Outside the Lines." Last year, the figures fell 4 percent. Officials said they do not have statistics for the 2013 season but expect that participation rates will be flat.

The decline in participation is not reflected on Pop Warner's website, which boasts that "participation has steadily increased to today's record numbers," adding that "over 250,000 youths participated in Pop Warner-sanctioned football programs in 2010, and those numbers are continuing to grow."

Representatives from Pop Warner and USA Football suggested the reasons for the drop in participation were unclear and could be attributed to several factors. Among those cited were the nation's economy and the ongoing trend of youth specializing in a single sport. Officials pointed to a survey showing declines in participation in other team sports, including baseball and basketball, although dips among core participants were not as severe.

The downward trends in youth football participation coincide with a series of ominous reports about football and brain damage in the NFL. In 2005, the first of dozens of confirmed cases of former NFL players with neurodegenerative disease was reported. In 2009, Congress held hearings on the NFL's long-standing efforts to conceal the connection between concussions and mental illness. In 2010, a league spokesman acknowledged for the first time a connection between concussions and "long-term problems."

In early 2011, former Chicago Bears defensive back Dave Duerson committed suicide by shooting himself in the chest to preserve his brain for study. He was later diagnosed with Chronic Traumatic Encephalopathy, or CTE, a brain disease that has been linked to football.

Tony Strickland, an associate clinical professor of neurology at UCLA's Geffen School of Medicine who sits on the Pop Warner's Medical Advisory Committee, said he believes participation dropped "in part because of the description of individual cases and the information out there about the incidence of CTE. If I'm a parent, anybody hearing that information, in the absence of other science, would be foolish not to be cautious."

But Strickland, who is CEO of the Sports Concussion Institute, said concerns about football and head injuries have outstripped the pace of scientific evidence, creating unwarranted hysteria about the risks of playing football.

"I have felt that the pendulum swung way ahead of the science and what we know," Strickland said.

In 2012, Pop Warner significantly cut back on the amount of tackling permitted during practice. This year, the organization announced a partnership with the NFL to endorse "Heads Up" football, a program launched by USA Football and designed to teach proper tackling techniques to minimize head contact.

In the coming months, Pop Warner's Medical Advisory Committee is expected to take up more rule changes that some officials believe could foreshadow where football is headed, as parents and players continue to reassess the risks.

One proposal under consideration would take all linemen out of the three-point stance. Bailes, its chief proponent, said requiring players to start upright would cut down on head-to-head collisions that can lead to brain injuries.

Bailes co-authored a study in the November issue of the Journal of Neurosurgery showing how repeated "subconcussive impacts" can lead to brain damage. One way to minimize those impacts in football, the study concluded, might be to have linemen start in a "squatting position, to remove them from the inexorable, ubiquitous, gratuitous head contact on every play."

"I really think we need to limit the number of head impacts," said Bailes, who also leads the department of neurosurgery at NorthShore Neurological Institute in Evanston, Ill. "I think that's where the sport needs to go."