Tag Archives: genetic basis for gender dysphoria

Gender Identity Disorder and Anorexia Nervosa in Male Monozygotic Twins – Review

This is a fascinating study of identical twins; one had gender dysphoria and one did not. Both twins developed anorexia.

Both twins were feminine in behavior from a young age and both were sexually attracted to men. Both had a difficult childhood with an abusive father.

Both twins were underweight at birth and needed intensive care. Both had developmental delays.

However, one twin considered himself to be a gay man while one identified as a straight woman.

In this case study, gender dysphoria did not cause the eating disorder.

This case highlights the importance of other factors in eating disorders, including genes, hormones, and trauma.

It raises the question; how important is gender identity as a cause of eating disorders?

This case is different from other case studies where gender dysphoria seems to be intimately linked to the eating disorder.

We can’t look at these two patients and conclude that gender dysphoria never contributes to eating disorders. However, this case is a good reminder to be cautious about drawing conclusions from other case studies. Perhaps there are just some people with eating disorders who also have gender dysphoria. Or perhaps there is some other factor which causes both eating disorders and gender dysphoria.

As always, we need more studies.

More about the Patients:

Eating Disorders

Twin A was diagnosed with AN-purging subtype and Twin B was diagnosed with AN-restricting subtype.

Twin B developed an eating disorder at an earlier age, but Twin A was more underweight and had a more disturbed perception of his body. Furthermore, Twin A was hospitalized for his eating disorder and Twin B was not.

Neither twin seems to have been able to maintain a healthy weight.

At age 16 Twin A “was admitted to a children’s hospital because of AN. Later, he was hospitalized in the psychiatric inpatient unit for adolescents. At first, his eating behavior was restrictive. Then he reported intermittent vomiting (AN binge-purge). His weight decreased to 46 kg/1.79 m (body mass index [BMI] ¼ 14.3 kg/m²). His ideal weight was 44 kg according to a BMI of 13.7 kg/m² , which shows his severe disturbance in body perception. During hospitalization, his behavior was sometimes aggressive. He was emotionally unstable, depressed, and was rarely able to engage in stable relationships. Despite strict dietary rules, he achieved a maximal weight of 55 kg (BMI ¼ 17.2 kg/m²). Soon after being discharged, his weight decreased again.”

Twin B’s eating disorder began at a younger age. “In puberty, he developed severe underweight. At the age of 13, he was 42 kg/1.58 m (BMI ¼ 16.8 kg/m² ). When he was referred to our outpatient unit at the age of 18½ years [for gender dysphoria], his weight was 48 kg and his height was 1.76 m (BMI ¼ 15.5 kg/m² ). He denied deliberate dieting, binging, or purging. Although he regarded himself as too slim, he did not manage to gain weight. Further medical checkups revealed no somatic cause for his underweight. An osteodensitometry yielded an osteopenia of the spine.”

Gender Identity

Twin A was a gender non-conforming gay male:

In childhood, he preferred girls’ games and toys (Barbie dolls) and was very close to his twin brother. His sexual feelings were always for males. Although he started cross-dressing at the age of about 16 years, his gender identification was always male. He considered himself to be a homosexual.”

Twin B was a trans woman:

“As far as he could remember, he had felt he was a girl, preferring girls as playmates and had started cross-dressing at nursery school. In gymnastic lessons, he refused to change with the other boys because he was ashamed of his body. Eventually, he refused to attend sports lessons at all. When he was 9 years old, he started to grow his hair. His class mates seemed to accept him as a girl. When he started to work as a hairdresser, he tried to correspond to the male gender role and did not cross-dress. However, at his professional school and in his free time, he continued to cross-dress. His employer, who realized he was transsexual, permitted and encouraged him to cross-dress at work, which consequently allowed him to live as a young woman. Sexually, he was always attracted to men. However, in contrast to his brother, he never considered himself to be homosexual and viewed this attraction as ‘‘heterosexual.’’ Until this point, he had not engaged in sexual relationships either with men or with women.”

Twin B requested hormonal and surgical sex reassignment.


The twins grew up together in a small Swiss city without any other siblings. Their childhood was not easy:

“[Their father] was very authoritarian. He could not accept the sexual orientation and the cross-dressing of his sons and threatened them with assault and even with death.

…In family conflicts, [their mother] took a position between her husband and her sons. At a family consultation, she appeared emotionally unstable.”


The birth was a difficult one. Both twins were underweight and spent time in intensive care.

“the mother had been admitted to a hospital with hypertension, edema, and proteinuria at 38 weeks of gestation. The vaginal delivery was induced because of maternal preeclampsia. Twin A weighed 2.17 kg at delivery and his Apgar score was 9/9/9. Because of perinatal acidosis and hypotonia, he was kept in the incubator for 3 days. He was diagnosed with a subependymal hemorrhage with ventricular invasion. Twin B’s birth weight at delivery was 1.95 kg and his Apgar score was 7/9/9. Both twins were admitted immediately to the neonatal intensive care unit.”

Developmental Delays

They both had developmental delays:

“In early childhood, Twin A showed a developmental delay in language and motor skills and had deficits in cognitive and verbal skills. He was socially isolated and his behavior was often aggressive.”

“…Twin B had delays in language and motor development during early childhood. He showed the typical symptoms of attention deficit and hyperactivity disorder. The parents refused further assessment and treatment.”


Twin A was diagnosed with borderline personality disorder and subnormal verbal intelligence.

Twin B was diagnosed with gender dysphoria.

There is no obvious pattern to any of this. Twin A was larger at birth, but had more problems right after birth. Both had developmental delays, and Twin B may have had ADHD as well. Both were feminine in their behavior, but only Twin B developed gender dysphoria. Both were sexually attracted to men. Twin B developed an eating disorder earlier, but Twin A’s eating disorder seems more severe. Twin A has borderline personality disorder and Twin B does not.


The authors offer two possible hypotheses about the twins’ gender identity.

Perhaps the twins are on a continuum of gender non-conformity where gender dysphoria is at the extreme end.

Alternatively, perhaps gender dysphoria* in childhood is inherited, but the later development of gender identity is determined by environmental factors and psychiatric comorbidity.

“In childhood, both Twin A and Twin B showed gender atypical behavior and stereotypical feminine traits and interests. In adolescence, their sexual orientation was revealed to be homosexual. Twin A developed effeminate homosexuality with male gender identity, whereas Twin B stabilized his cross-gender identity. Although Twins A and B are concordant for GID in childhood and sexual orientation on a categorical level, they are now discordant for TS. On a more dimensional level, one could argue that Twins A and B show an opposite sex-dimorphic behavior and that they arrived at different points of a continuum. The fact that GID in childhood is a predictor for later homosexuality and TS could support the dimensional view. It could be hypothesized that GID in childhood is mainly hereditary, whereas the development of the later phenotype of the gender identification is determined by environmental factors and psychiatric comorbidity, as any difference between MZ twins provides strong evidence for the role of environmental influences.”

The authors also discuss the relationship between gender and eating disorders. However, they don’t address the fact that the two twins had different gender identities, but both had eating disorders.

Perhaps both gay men and trans women are vulnerable to eating disorders for different reasons, but perhaps genes, hormones, and environment matter more than gender identity.

“Homosexual men seem to have an increased vulnerability to eating disturbance and body dissatisfaction (Williamson & Hartley, 1998), are more dissatisfied with their weight (French, Story, Remafedi, Resnick, & Blum, 1996), and are more concerned about their attractiveness (Siever, 1994). Male AN is associated with disturbed psychosexual and gender identity development, which supports the hypothesis that males with atypical gender role behavior have an increased risk of developing an ED (Fichter & Daser, 1987). Furthermore, feminine gender traits are discussed as a specific risk factor for ED in men and women (Meyer, Blissett, & Oldfield, 2001). Although the role of sexual orientation as a risk factor for ED is well documented, there is hardly any literature about GID and ED. For men with disturbance of gender identity in addition to the aforementioned factors concerning sexual orientation, underweight could be a way to suppress their libido and the expression of their secondary sexual characteristics and, at the same time, correspond to a female ideal of attractiveness (Hepp & Milos, 2002).”

We need more research!

“Further research in eating behavior and body dissatisfaction in patients with GID could provide more insight into the role of gender identity in the development of ED and lead to a better understanding of ED as well as GID.”


* In this case, gender non-conformity might be a more fitting phrase. Twin A does not seem to have ever wanted to be a girl.


Original Source:

Gender Identity Disorder and Anorexia Nervosa in Male Monozygotic Twins by Urs Hepp, Gabriella Milos, and Hellmuth Braun-Scharm in Int J Eat Disord. 2004 Mar;35(2):239-43.


Identical Reared apart Twins Concordant for Transsexuality – Review

This is a brief article about a case of identical twins reared apart who both had gender dysphoria. Unfortunately, it is based on an interview with only one of the twins. This makes it a suggestive report, but not scientific proof.

In addition, one of the twins was born with an intersex condition. This raises the question of whether his gender dysphoria was caused by genes or the prenatal environment.

The twins were born male and separated at birth and raised in different families. They did not meet each other until they were 15. DNA testing confirmed that they were identical twins.

This is only one case, so the conclusions we can draw are limited.

The main weakness of the report, however, is that it is based on the memories of only one of the twins, LT. The authors were able to interview LT in his 50s.* It is possible that his memories of meeting his twin AT 35 years ago would be incorrect or might exaggerate similarities or differences.

According to LT,

“Prior to meeting, by age 8 years both twins experienced gender discomfort, engaged in cross-dressing, and felt that they should have been born as the other gender. Also prior to meeting, both twins experienced unease with the anticipated and actual secondary sexual development of puberty. Furthermore, unbeknownst to his twin, at age 14 years LT was fully committed to undergoing sex reassignment surgery and so convinced his mother that she took him to see a urologist.”

According to the author of the report, this means that both twins met the criteria for gender dysphoria, “persistent cross-gender identification and a strong desire to change the sexual characteristics to those of the other gender.”

I am not comfortable with diagnosing someone you have not interviewed. Clearly, AT had some issues with his gender, but how did he define himself? Did he want to have surgery? Did the twins continue to have contact and did AT’s gender dysphoria persist into adulthood? Did they influence each other after age 15?

This report is also about a tragedy. AT committed suicide at age 35.

The twin’s birth mother had skin cancer and doctors believed she should give one twin up for adoption. AT was therefore put in a state-run institution and then adopted by a less well-to-do family. His parents were “religious, punitive, and rejecting of his cross-gendered behaviors.” (Presumably we know all this from his twin, LT.)

LT was raised by his biological parents. His family was more financially comfortable. His mother and sisters “supported his female identity as it developed and afterward.”

The authors of the study stress the importance of family support in preventing suicide, but there are a number of other factors that might have also influenced AT. Adoptees are more likely to attempt suicide than non-adoptees. It might also be upsetting to learn that you had a twin and your birth parents had kept them, as AT did. Having parents with a lower socio-economic status also increases the risk of suicide. AT was therefore more at risk for suicide to start with, although we also know that family support reduces the risk of suicide for people with gender dysphoria.

An interesting aspect of this study is that LT was a “sickly infant” and had hypospadias, a birth defect in the urethra. The Intersex Society of America lists it as an intersex condition. There is no information in the study as to whether or not AT also had hypospadias or any other intersex condition.

Did the hypospadias contribute to LT’s gender dysphoria? Could having a scar on your penis influence feelings about gender identity?

Could the hypospadias be a sign of something else that also caused LT’s gender dysphoria?

According to Wikipedia, the cause of most cases of hypospadias is unknown, but it may be influenced by: having an older mother, the mother taking progesterone during pregnancy, the fetus not producing enough testosterone or nor responding to it, or genetics. It seems reasonable to ask if these factors might also affect gender identity.

In this case, the mother was 40 when the twins were born, which might have been a factor in LT’s hypospadias. We don’t know about any hormonal factors.

We have no way of knowing what caused LT’s hypospadias, but it does raise the question of whether the twins’ issues with gender were caused by a genetic factor or something to do with the prenatal environment or the genital surgery.

I am frustrated by this study. It seems like a good case for a genetic component to gender dysphoria – twins reared apart are a classic test of genetic effects. Unfortunately, since it is based on the account of one twin, the evidence is not as strong. It might be that the twin who lived developed a narrative to make sense of his twin’s tragic suicide.

In addition, the fact that one twin had an intersex condition at birth might be a sign of something unusual in the twin’s prenatal environment.

Mostly, though, you can’t say you have scientific proof that the twins were alike if you only interview one of the pair of twins.

It is so very hard to get good data on gender dysphoria. People with gender dysphoria are extremely rare and twins with gender dsyphoria are even rarer. Add in a much-too-high suicide rate, and you have even less ability to collect data.

Once again, we need more research in this area. As Heylens et. al said in their review of the literature: “detailed registers of GID twins, preferably on MZ twins discordant for GID and DZ twins are needed, to gain more decisive information about the influence of genetic vs. environmental factors in the development of GID.

Original Case Report:

Identical Reared apart Twins Concordant for Transsexuality by Nancy Segal & Milton Diamond, Letter to the Editor, published in: Journal of Clinical and Experimental Medicine, April 2014 Volume 6, Issue 2, Page 74.

*The article refers to the twins with male pronouns. The article does not make it clear what the twins did when they grew up. LT, if you have transitioned and I am using the wrong pronouns, I apologize.

More about twins and gender dysphoria:

My review of “Gender Identity Disorder in Twins: A Review of the Case Report Literature.”

Study of Gay Brothers Suggests Genetic Basis of Male Homosexuality – Discovery Magazine Article

An interesting and important piece from Discovery magazine.

“Are people born gay or is it a choice? A new study of gay brothers, the largest to date, adds more scientific evidence that there’s a genetic basis for homosexuality.

A genetic analysis of over 409 pairs of gay brothers found that two areas of the human genome, a portion of the X chromosome and a portion of chromosome 8, were associated with the men’s sexual orientation. The findings gel with a smaller study conducted in 1993 that implicated the same area of the X chromosome.”

You can read the rest of the article at Discovery magazine.

So why is this important for research on gender dysphoria?

1) If sexual orientation is influenced by genes, then researchers looking for genes related to gender identity need to control for sexual orientation.

Trans men (born female) are usually attracted to women and about half of trans women (born male) are attracted to men, so they might share genes with cis lesbians or gay men.

Future studies of genes and gender dysphoria need to include cis gay men and lesbians in the control groups.

2) The genes that may be involved in male homosexual orientation were found on the X chromosome and chromosome 8. The researchers looked at the whole genome for 409 pairs of homosexual brothers.

Studies of genes for gender dysphoria have focused on genes known to be related to sex hormones and the X and Y chromosomes (read more in Genes and Gender Dysphoria). This makes sense if you are looking at behavior that is related to sex differences, but perhaps the genes are somewhere else.

So far, researchers have had not luck finding genes related to gender dysphoria in trans women and only some luck finding genes related to gender dysphoria in trans men. Perhaps the genes for gender dysphoria and the mechanism involved are not what we expect.

A whole genome scan for genes related to gender dysphoria would be a great study for someone to do.