The Boxing Brain

Boxing and the Brain
It has long been established, and is widely accepted within the sport, that boxing can result in "punch-drunk"  syndrome, in addition to which there are wide-ranging effects of boxing on the brain, but what do we know from the scientific evidence?
Research has shown that the force of a professional boxer's fist is equal to being it by a 6kg bowling ball travelling at 20 miles per hour. This is 52x the force of gravity. To put this into context, a quick search on the internet will tell you that a g-force equivalent to around 50 g is likely to cause death. To put this into context, Apollo 16 created a g-force of 7.19g on re-entry into the earth's atmosphere.
The Journal of Combative Sport found that between January 1960 and August 2011, there were 488 boxing-related deaths, of which 66% per linked to injuries of the head and spinal cord. A more recent estimate by Forstl et al., (2010) agreed with this, showing that there have been around 10 deaths per year in boxing since 1900, however it's important to consider that these numbers are based on acute injuries, whereas if deaths attributed to long-term diseases associated with boxing such as dementia, were included, the rate would likely be much higher. 
A range of research studies have shown that between 15% and 40% of ex-boxers have symptoms of chronic brain injury (that is, persistent injuries over the long-term). More recent research has shown that nearly all professional boxers have some degree of brain damage, (even if they do not have obvious symptoms).
Crucially, the scientific evidence has shown that often difficulties are not detected on brain scans, but that assessment of thinking skills ("neuropsychological assessment") can detect the early warning signs of change in the brain. 

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Neuropsychology of Boxers

Studies have shown that planning, attention and concentration and memory are cognitive "thinking" skills that are areas of weakness for boxers (McLatchie et al., 1987)
There is evidence to suggest  that boxers' cognitive skills can deteriorate not only as a result of trauma during a fight, but also leading up to a fight because of sparring, rapid weight loss and pre-bout anxiety (Ravdin et al., 2003).
Following a bout, it is assumed that there is a period of recovery after which, the brain settles down - even if it hasn't had an injury or a concussion, it is shaken repeatedly during a fight. Research has shown that boxers with more bouts in their fighting histories (more than 12) had greater impairment in their thinking skills during this period. This suggests that the more seasoned boxer, experiences poorer brain recovery after a fight (Ravdin et al., 2003).
There is now consistent evidence that the higher the number of bouts a fighter has participated in, the worse their cognitive skills are, thus indicating a greater degree of brain damage (this is the case even when there has not been an obvious injury or concussion). This means that a boxer may have had no incidents to suggest a brain injury, but nonetheless their brain is different, impacting how it works. In addition to this, number of losses and number of draws are also related with poor cognitive and therefore, brain health (Drew et al., 1986; Stewart et al., 1994).
These investigators also found that
boxers with the most professional experience (i.e., more than 12 bouts) had some evidence of poor neurocognitive performance
during the presumed recovery period. Other researchers reported that both the number of bouts and number of losses plus draws
correlated with cognitive deficits in professional boxers (Drew et al., 1986; Stewart et al., 1994). In addition, increased sparring
exposure was related to worse neuropsychological performance, primarily on tests involving attention, concentration, and
memory (Jordan et al., 1996). Thus, degree of sparring exposure, rather than the number of actual competitions, appears to
be associated with worse cognitive functioning in some boxers.
Studies of the neuropsychological affect of amateur boxing revealed few, if any, neurocognitive deficits (Moriarity et al.,
2004; Porter, 2003; Porter & Fricker, 1996; Stewart et al., 1994; Timm, Wallach, Stone, & Ryan, 1993). Moriarity and other
researchers (2004) examined amateur boxers participating in multiple bouts during a 7-day tournament and found that, with the
exception of those whose contests were stopped by the referee, there was no evidence of cognitive dysfunction in the immediate
post-bout period. Porter (2003) followed 20 amateur boxers over a 9-year period and found no evidence of decreased neurocognitive
test performance. In fact, the boxers evidenced significantly better performance over time than age-matched controls.
Porter and Fricker (1995) found no evidence of neuropsychological impairment in amateur boxers compared with controls, and
there was no association between neuropsychological performance and boxing exposure. Timm and colleagues (1993) examined
boxer safety and concluded that serious injuries occurred very rarely and that amateur boxing is generally a safe sport.
When neurocognitive deficits have been evident in amateur boxers, they occurred primarily on tasks measuring attention,
concentration, memory, and motor speed (Brooks, Kupshik,Wilson, Galbraith, &Ward, 1987). Those amateur boxers with the
most ring experience showed greater adverse effects, but the magnitude of their deficits was considered mild, relative to normative
control groups. Compared with other examination procedures (e.g., neurologic exam, computed tomography scan, electroencephalogram),
neuropsychological tests are the most sensitive method for detecting neurological dysfunction in active
amateur boxers (Ross et al., 1983). This line of research has led some investigators to conclude that carefully controlled durations
of amateur boxing may indeed be neuropsychologically safe (Butler et al., 1993). One limiting factor associated with
boxing studies, however, is participant self-selection; that is, boxers who decline to participate in such research may differ
in important ways from those who agree to participate.
Stewart and colleagues (1994) found statistical trends between the total number of bouts fought prior to a baseline exam and
changes in memory, visual-constructional ability, and perceptual-motor ability over time. No significant associations were
found between the number of bouts that occurred after the baseline assessment and neurocognitive abilities, or between
number of bouts or sparring exposure and other outcome measures such as brain stem auditory-evoked potentials and

Formal Boxing Recommendations

Neurodegenerative Disorders

Neurodegenetative Disorders
Acute brain injuries are fortunately relative rare in boxing, however research has attributed head injuries of this nature to around 80% of the, on average, 10 deaths per year in boxing, since 1900 (Forstl et al., 2010)

Concussion

Post-Concussion Syndrome
This is diagnosed when there are persistent symptoms of a concussion, beyond what might be normally expected. Heilbronner et al. (2009) explained that some boxers (mostly those with the more extensive fight histories) are at risk of not recovering fully from a concussion.  There is currently research around whether post-concussion syndrome can develop into CTE dementia in boxers and other athletes.This is diagnosed when there are persistent symptoms of a concussion, beyond what might be normally expected. Heilbronner et al. (2009) explained that some boxers (mostly those with the more extensive fight histories) are at risk of not recovering fully from a concussion.  There is currently research around whether post-concussion syndrome can develop into CTE dementia in boxers and other athletes.

Psychitric Conditions

Neurodegenetative Disorders
Acute brain injuries are fortunately relative rare in boxing, however research has attributed head injuries of this nature to around 80% of the, on average, 10 deaths per year in boxing, since 1900 (Forstl et al., 2010)