For most people there is congruity between their physical sexual identity and their psychological sense of their sex. Gender is the term used to describe the latter. Generally, men identify as men and women as women and when there is a disconnect between physical sex and psychological gender, the person has a gender identity disorder.
or many this is experienced as problematic and distressing and is referred to as gender dysphoria. Those who seek remedies to enable them to transition to their psychological gender are known as transsexuals.
There is continuing controversy about this condition and its treatment, particularly since it involves surgery that many see as mutilating.
Yet follow-up studies after surgery indicate that the many mental health problems such people experience are resolved.
Some high-profile cases of gender reassignment in recent years have spoken of their experience. The most recent is Caitlyn Jenner, the Olympian formerly known as Bruce Jenner. The recent focus internationally on transgender people is also matched by controversy about how common it is.
The Diagnostic and Statistical Manual 5th edition, the American guide to psychiatric disorders, quotes a prevalence of five to 14 per 100,000 for those born male and two to three per 100,000 for those born female.
The HSE on its website says the prevalence is one in 4000 while a 2013 study of veterans published in the American Journal of Public Health Public identified a rate of 22.9/100,000.
The problem with these figures is that some are based on those attending for treatment, while others refer to those who show some, but not all, features of the condition. There have been no prevalence studies in the general population.
While transgender adults and treatments, including hormonal and surgical interventions, have been recognised for decades, the presence of gender dysphoria in children and adolescents has only recently come to prominence. The lesser problem of cross-dressing or opposite sex behaviours in children has long been recognised.
In a study of childhood behaviours in four to 18-year-olds in the 1970s, mothers reported that 2-4pc of boys and 5-10pc of girls in the general population behaved as a member of the opposite sex from time to time while the wish to be a member of the opposite sex was much less common.
Among adolescents specifically, 5-13pc of teenage boys and 20-26pc of teenage girls in a non-clinical sample reported cross-gender behaviour while 2-5pc of boys and 15-16pc of girls reported sometimes desiring to be the opposite sex.
In spite of this, very few presented for evaluation or treatment even in countries such as the Netherlands where there is a well-established formal program.
Clearly there is evidence of gender experimentation. So it is incorrect to assume that transgender behaviour necessarily indicates that the young person has gender dysphoria, as gender identity disorder is now known.
A controversy relates to the use of hormone treatments to suppress puberty in those who may have gender identity disorder. Writing in the journal Pediatrics (2012), Walter Meyer points out that International Guidelines explicitly state that pre-pubertal children should not try to change gender.
He goes on the write "Many of the presentations in the public media concerning childhood GID give the impression that a child with cross-gender behaviour needs to change to the new gender or at least should be evaluated for such a change.
"Very little information in the public domain talks about the normality of gender questioning and gender role exploration and the rarity of an actual change."
In these circumstances he supports the use of hormonal treatment to delay puberty as a way of preventing the major physical changes that occur to the body at this time so that a careful decision can be reached about gender reassignment treatment without the concerns that major body alternations have occurred, making surgery, should it be indicated, more difficult.
On the other side are those who argue that while hormones will delay puberty, should the final decision be that the child does not have the condition and they should go through puberty, their body may not develop normal secondary and tertiary sex characteristics.
In such a controversial area, with so little knowledge, parents have immense trouble knowing how best to proceed. The rarity of the condition means that they will have few to turn to who have had a similar experience.
Their friends and family are unlikely to be able to offer any insights apart from being available to listen. Some parents will want to try and resolve the problem as quickly as possible, by requesting physical assessments to identify medical causes.
Others may be too fearful, distressed or stigmatised to speak to any confidants about it, and may be forced to rely exclusively on the clinical analysis of the medical profession
But of some comfort is the recommendation that for pre-pubertal children, the clear advice is to wait and see. Most children do not proceed to medication or surgery and the behaviour will resolve.
For adolescents who are post-pubertal and wish to proceed with cross-sex hormone treatment, parents are likely to experience even stronger emotions.
Rewriting the historical memories of the cuddly little boy (or girl), who is now a woman (or man) is confusing and painful.
Incomprehensibility at why he now wants to be a girl and excessive self-blame and guilt that their parenting may have had a role, haunt many.
Some cut off ties with their child. For others the basic question of the likely outcome is to the fore.
Will surgery help or harm? The fruitless wish to turn back the clock to the happy days of childhood innocence is understandable.
Parenting a young adult who wants gender reassignment is a test of parents' unconditional love and at time like this, their child needs it more than ever.