Something is happening at the end of the wars in Iraq and Afghanistan that mental health experts are finding hard to explain: British and American soldiers appear to be having markedly different reactions to the stress of combat. In America, there has been a sharp increase in the number experiencing mental-health problems, including post-traumatic stress disorder (PTSD). Between 2006 and 2007 alone, there was a 50 per cent jump in cases of combat stress among soldiers and suicides more than doubled. Why the precipitous rise? And why hasn't there been an accompanying rise in these symptoms among British troops?

The conclusion that British soldiers appear to have a different psychological reaction to the stresses of these modern conflicts was the finding of several recent high-profile studies. This year, in a Royal Society journal, Neil Greenberg of the Academic Centre for Defence Mental Health at King's College London and colleagues reported that studies of American soldiers showed PTSD prevalence rates of in excess of 30 per cent while the rates among British troops was only four per cent. UK soldiers were more likely to abuse alcohol (13 per cent reported doing so) or experience more common mental disorders such as depression (20 per cent).

Such differences were found even when comparing soldiers who served in the most intense combat zones. In addition, while researchers found increased mental-health risk for American personnel sent on multiple deployments, no such connection was found in British soldiers.

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One theory to explain these differences is that the minds of soldiers are responsive to cultural expectations of how they should feel – and that those expectations can be different from one place (or time) to another. One theory to explain these differences is that the minds of soldiers are responsive to cultural expectations of how they should feel – and that those expectations can be different from one place (or time) to another.

"Despite some claims to the contrary," Greenberg et al write, "PTSD seems not to be a 'universal stress reaction', arising in all societies across all time. Evidence from both world wars suggests that the ways in which service personnel communicate distress is culturally determined and that the development of PTSD may be one more phase in the evolving picture of human reaction to adversity."

This suggests that the psychological reaction to war does not happen in a flash like a shrapnel wound. Rather, it evolves as the soldiers integrate their experiences with the values and expectations of their culture. British soldiers in the Boer Wars were likely to complain of joint pain and muscle weakness, a condition their doctors called "debility syndrome". In the US Civil War, soldiers often reacted to the trauma of battle by experiencing an aching in the left side of the chest and having the feeling of a weak heartbeat, labelled "Da Costa's syndrome". In the First World War, soldiers experienced "shell shock", with symptoms that included nervous tics, grotesque body movements, and physical paralysis. It was not until after the Vietnam war that soldiers began to describe their symptoms primarily in terms of the intrusive thoughts, memory avoidance and uncontrollable anxiety and arousal that makes up the core of the PTSD diagnosis.

The simple but mind-bending truth is that mental illnesses such as PTSD can be both culturally shaped and utterly real to the sufferer.

There is evidence, in short, to suggest that American and British soldiers come home to significantly different expectations for their psychological recovery and those expectations matter a great deal. In America, soldiers frequently return to a culture that fully expects them to be psychologically wounded by the experience.

Diagnosis of PTSD began to take shape in the US after the Vietnam war and represents much more than a clinical set of symptoms. It has become a world view; a weapon in a battle between a militaristic view of the world – where going to war and using deadly force can be both morally justified and personally uplifting – and a therapy view of the world, where violence is an aberration that inevitably damages the human psyche and spirit.

Originally called post-Vietnam syndrome, modern PTSD began in hothouse rap sessions held by Vietnam Veterans Against the War and supervised by antiwar psychoanalysts. The motivations behind the creation of the diagnosis are clear in early descriptions of post-Vietnam syndrome such as this one written by a young psychoanalyst named Chaim Shatan and published in the New York Times in the spring of 1972: These veterans were suffering because they had been, "deceived, used and betrayed" by both the military and society at large. That the creation of this syndrome would help the anti-war effort was clear. Shatan wrote in a memo to his colleagues at the time: "This is an opportunity to apply our professional expertise and anti-war sentiments."

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The diagnosis of post-Vietnam syndrome was intended to highlight the psychological cost of participating in what many mental-health providers perceived to be an unjust war.

A generation later, the concept of PTSD at first appears to have come a long way from post-Vietnam Syndrome with its overt anti-war meaning. It has become much more clinical and de-politicised to the point where the military itself now recognises and employs the diagnosis. But American trauma counsellors still constitute a population with a direct intellectual lineage to the anti-war psychiatrists who forged the PTSD diagnosis. In their rush to help American soldiers, these healers often carry with them anti-war sentiments and other cultural assumptions about how the experience of war will manifest itself in psychological injury.

As several historians have pointed out, Vietnam veterans didn't return from that conflict with their PTSD symptoms fully expressing themselves. Many came home believing they were psychologically healthy, only to succumb – sometimes decades later – to cultural expectations that they were indeed suffering from the trauma of the experience. Vietnam veterans applying for PTSD disability doubled between 1999 and 2004.

"This raises the possibility of post-combat 'belief, expectation, explanation, and attribution'," wrote Dr David Marlow in a Rand report. "Participation in Vietnam caused veterans to see themselves and to be seen as a population suffering from a host of psychological symptoms. The public's view of them increased their stress, exacerbating whatever problems may have already existed." The reason that veterans have been prone to becoming psychological casualties, Marlow and others suggest, is because their unconscious minds were responding to a culture and a PTSD-focused mental-health community that predicted this turn of events.

This sort of cultural expectation has not yet fully taken hold in the UK where many remain resistant to a conception of the human mind as fragile in the face of trauma. Indeed many UK scholars and mental-health providers have pushed back at the spread of this American diagnosis.

"In a momentous shift, contemporary Western culture now emphasises not resilience but vulnerability," laments Derek Summerfield, of King's College, who has worked extensively with victims of war and genocide. "We've invited people to see a widening range of experiences as liable to make them ill."

Humans have suffered a great deal throughout history and have developed myriad cultural beliefs to sustain themselves in the aftermath of horror. PTSD appears particularly weak in this light. By isolating trauma as a malfunction of the mind that can be connected to discrete symptoms and targeted with specialised treatments, the disorder removes trauma from other cultural narratives and beliefs that might give deeper meaning to suffering. It claims to be value-neutral to cultural beliefs but this is problematic, given that those beliefs – be it God's plan for someone who's lost a child or patriotism for the soldier crippled in battle – are the very places where we once found solace and strength.

As it evolved away from Post-Vietnam Syndrome and into its modern clinical form, PTSD left behind the quests for social meaning in tragedy. In doing so, it has set adrift those struggling in the aftermath of trauma. In contrast to those angry but socially engaged Vietnam War veterans, the personal accounts of current-day US soldiers returning from Afghanistan and Iraq often seem pigeonholed into a PTSD diagnosis that is tied to a particularly modern style of lonely hyperintrospection.

The frustration, anger, and unhappiness of modern soldiers have been moved from the social (where one might find moral anger, nationalistic justification, or religious meaning to justify the sacrifice) to the biopsychomedical. Because the disorder focuses largely on internal states and chemical imbalances within the individual brain, this explanation for psychological problems often leaves the soldier – to borrow a recent US military marketing slogan – feeling like "an army of one".

Patrick Bracken, of Bradford University's Dept of Health Studies, argues that the emergence of PTSD is a symptom of a troubled postmodern world. "In most Western societies there has been a move away from religious and other belief systems which offered individuals stable pathways through life, and meaningful frameworks with which to encounter suffering and death," Bracken writes. "The meaningful connections of the social world are rendered fragile."

Although we might be able to ignore the absence of these belief systems during our normal day-to-day lives, truly traumatic events have a tendency to startle us into awareness of a heart-stopping emptiness. The diagnosis of PTSD can categorise some of our reactions to trauma, but in the end it is cold comfort. It cannot replace what we've lost.

Ethan Watters is the author of 'Crazy Like Us: the Globalisation of the Western Mind', published by Robinson. (£9.99). To order a copy (free P&P) call Independent Books Direct on 08430 600 030, or visit independentbooksdirect.co.uk