In his 2010 book, Webbe suggested two alternative explanations for the inconsistent findings of these studies:
First, that heading-related neurocognitive impairment may simply not be very "robust," a hypothesis supported by (a) data demonstrating that the forces that occur during heading are typically less than would be predicted to cause concussions; (b) anecdotal evidence that the vast majority of adult soccer players appear to remain cognitively intact during and after their playing careers, and (c) the absence of known risk factors (other than players putting themselves in a position to challenge for the ball) that would predict who would be adversely affected by heading and when;  or
Second, that the deleterious effects of heading upon neurocognitive functioning may represent the interactions in repetitive sub-concussive trauma of unknown factors unique to the individual, a hypothesis supported by data from studies showing that soccer players most likely to exhibit lower levels of neurocognitive functioning (and/or, in the case of the Albert Einstein study, exhibiting changes in the white matter of the brain and impaired memory) have been those who headed at relatively high frequencies.  Again, this is what the Einstein study found.
As Webbe suggests, and, as the Einstein researchers appears to confirm, "it may well be that susceptibility to the minor blows associated with heading in soccer players is not a graded phenomenon that distributes across all individuals, but rather is not only idiopathic [i.e. of unknown origin] but also somewhat dichotomous." In other words, as the Einstein study suggests, "some individuals may have significant heading-related difficulties, while others may have none."
As Webbe notes, that this is so should be "no surprise" because identifying the concussion-prone individual in advance is exceedingly difficult. Using American football as an example, two players can seemingly exhibit the same hit, but one suffers a concussion and the other doesn't, and we don't know why.
The bottom line, Webbe says, is that heading-related cognitive impairment due to repetitive sub-concussive blows may occur in relatively few individuals, but, whatever the reason, the "clinical response is the same": to determine who is prone to injury from repetitive heading and intervene to prevent further damage."
Soccer heading in youth
As is so often the case in head injury research, pediatric studies on the effect of cumulative soccer heading on the brains of children are few and far between, with only a handful of studies providing data on heading in youth soccer. [24-26, 41-44]
Similar to the studies on adults, the outcomes are contradictory.  The issue, however, as Webbe notes, is "not trivial," citing a 2009 study of which he was a co-author  which found that almost 50% of a sample reported concussion-like symptoms (headache, dizziness, balance problems) after heading a soccer ball, similar to post-concussion reports from adolescent and Olympic soccer athletes. 
In addition, Webbe suggests that, as in adult soccer, politics regarding heading leads some coaches and parents to be reluctant to participate in studies at the youth level because it might "deter children from heading behavior and competitiveness in the sport."
A 2013 study of female middle school soccer players  sustained concussions at a rate higher than their high school and college counterparts, that most continued to play despite experiencing symptoms, and that less than half sought medical attention. (All arguments advanced by the SLI White Paper as supporting its recommendation for delaying heading in soccer).
SLI cites the study as support for its assertion that 30.5% of concussions "are caused by heading the ball or by attempting to head the ball and colliding with a player, object or the ground," but, first, it should be noted that such a percentage is not all that different from the 27.% caused by such mechanisms among high school athletes in a recent study (which suggests that middle school soccer players are not uniquely vulnerable to concussions occuring during the act of heading),  and, second, as at least one prominent researcher, Dawn Comstock, an epidemiologist at the Colorado School of Public Health who has studied extensively sports injuries at the high school level, notes, the study fails to provide a breakdown of how many of the heading-related concussions were from athlete-athlete contact versus contact with the ball versus contact with the ground. "This is disappointing, Comstock told MomsTEAM at the time the study was published, "since they had the data and just didn't present it," an omission that she felt was was "really important from a prevention standpoint: if we want to significantly reduce concussions in youth soccer, [we need to know] do we need to ban heading altogether, or would we be successful if rules prohibiting athlete-athlete contact during heading were enacted and strictly enforced?"
The 2010 AAP Clinical Report on youth soccer injuries  suggests - in what some might characterize as wishful thinking - that the "contribution of purposeful 'heading' of the soccer ball to both acute and potential long-term concussive effects, such as cognitive dysfunction, seems less controversial today, as previously."
At the time, the AAP concluded that a critical review of the literature "does not support the contention that purposeful heading contacts are likely to lead to either acute or cumulative brain damage, and additional study is necessary to provide confirmatory evidence of neuropsychological consequences of subconcussive soccer-related head contacts." That it reaffirmed the Clinical Report as recently as May 2014 suggests that the AAP does not view any of the studies published since the report was initially published in 2010 as warranting a change in its position.
A 2012 study  by researchers at the Cleveland Clinic concurred with the AAP, concluding, after reviewing theoretical concerns, the results of biomechanical laboratory experiments, and then-available clinical data regarding the effects of chronic, subconcussive head injury from soccer heading, that there were, in its words, "no support" in recent studies for the findings of the early Norwegian studies. Moreover, they asserted that the more recent studies failed to show that soccer players experience neurocognitive impairment compared with athletes in other sports, or that more frequent headers scored more poorly on sophisticated neuropsychiatric evaluation than less frequent headers.
This is exactly what a 2013 study by the Albert Einstein researchers  did find, although, to be fair, the Cleveland Clinic researchers were quick to include a caveat to their conclusions: that damage building up over the long term could not be ruled out. The authors thus said that players, parents, and coaches needed "to be aware that ... any possible detrimental effect from repetitive subconcussive heading may only become clinically evident decades into the future."