responsible for the symptoms was challenged by rigler, who suggested that the cause of the persisting symptoms was actually "compensation neurosis": the railroad's practice of compensating workers who had been injured was bringing about the complaints. later, the idea that hysteria was responsible for the symptoms after a mild head injury was suggested by charcot. controversy about the syndrome continued through the 20th century. during world war i many soldiers with puzzling symptoms after being close to a detonation but without any evidence of a head wound. the illness was called shell shock, and a psychological explanation was eventually favoured. by 1934 the current concept of pcs had replaced ideas of hysteria as the cause of post-concussion symptoms. british authorities banned the term shell shock during world war ii to avoid an epidemic of cases, and the term posttrauma concussion state was coined in 1939 to describe "disturbance of consciousness with no immediate or obvious pathologic change in the brain". the term postconcussion syndrome was in use by 1941. in 1961, h. miller first used the term "accident neurosis" to refer to the syndrome which is now called pcs and asserted that the condition only occurs in situations where people stand to be compensated for the injury. the real causes of the condition remain unclear. controversy though no universally accepted definition of postconcussive syndrome exists, most of the literature defines the syndrome as the development of at least three of the following symptoms: headache, dizziness, fatigue, irritability, impaired memory and concentration, insomnia, and lowered tolerance for noise and light. one complication in diagnosis is that symptoms of pcs also occur in people who have no history of head injury, but who have other medical and psychological complaints. in one study 64% of people with tbi, 11% of those with brain injuries, and 7% of those with other injuries met the dsm-iv criteria for post-concussion syndrome. many of these individuals with pcs were misdiagnosed as having other unrelated conditions due to commonality of symptoms. (see diagnosis above). headache is one of the criteria for pcs, but it is notably undetermined where the headache comes from. couch, lipton, stewart and scher (2007) argue that headaches, one of the hallmarks of pcs, occur in a variety of injuries to the head and neck. further, lew et al. (2006) reviewed ample studies comparing headaches to post-traumatic headaches and found that there is wide heterogeneity in the source and causes of headaches. they point out that the international headache society lists 14 known causes of headaches, as well. furthermore, the headaches may be better accounted for by mechanical causes, such as whiplash, which is often mistaken for pcs. an additional possibility is that posttraumatic stress disorder can account for some cases diagnosed as pcs, but for emotional regulation as well. depression, posttraumatic stress disorder, and chronic pain share symptoms resembling those of pcs. one study found that while people with chronic pain without tbi do report many symptoms similar to those of post-concussion syndrome, they report fewer symptoms related to memory, slowed thinking, and sensitivity to noise and light than people with mtbi do. additionally, it has been found that neuroendocrinology may account for depressive symptoms and stress management due to irregularities in cortisol regulation, and thyroid hormone regulation. lastly, there is evidence that major depression following tbi is quite common, but may be better accounted for with a diagnosis of dysexecutive syndrome. in a syndrome, a set of symptoms is consistently present, and symptoms are linked such that the presence of one symptom suggests that of others. because pcs symptoms are so varied and many can be associated with a large number of other conditions, doubt exists about whether the term "syndrome" is appropriate for the constellation of symptoms found after concussion. the fact that the persistence of one symptom is not necessarily linked to that of another has similarly led to doubt about whether "syndrome" is the appropriate term. a longstanding controversy surrounding pcs concerns the nature of its etiology – that is, the cause behind it – and the degree to which psychological factors and organic factors involving brain dysfunction are responsible. the debate has been referred to as 'psychogenesis versus physiogenesis' (psychogenesis referring to a psychological origin for the condition, physiogenesis to a physical one).