Nature, nurture, and disease transmission

4 min read

Bipolar disorder afflicts about 3% of people.  Half of them have a bipolar parent.

Therefore, what is the chance that a bipolar parent will produce a bipolar child?

The pendulum of belief about the causes of mental illness has swung from nurture to nature and back to the middle: a non-dualistic view that it’s both nurture and nature.  People used to describe mental illness as a character weakness or choice.  Then, we realized mental illness is highly heritable.  Now, psychiatric science is focused on how environmental factors activate genes.

In the case of schizophrenia, researchers have determined that childhood environmental stress increases the likelihood of manifesting mental illness more than having the gene does.  Childhood environmental stress includes things like abuse, neglect, brain damage, exposure to toxins, cannabis use, bullying, exposure to violence, or death of a parent.  Even having immigrant parents and living in a city are evidently associated with higher incidence and earlier onset timing (though specific causal relationship is unclear).  Genetics is not simple destiny.  Genes have to get turned on. 

That means that backward-looking data about heritability isn’t perfectly predictive of the future in individual cases.  Knowing about his own diagnosis and the mechanism of triggering gene expression, a bipolar parent can put concerted effort into preventing childhood adversity, and thus lowering the risk of “passing it on”.   Over time, the prevalence of mental illness among children of the mentally ill (C) could trend downward in the direction of overall population prevalence (A). 

Psychiatrists considering a bipolar diagnosis always ask if the patient’s parents (or other relatives) have bipolar disorder.  Inviting a patient to reflect on their childhood through that lens can generate an epiphany; it’s often observed that perhaps half of bipolar patients have a bipolar parent.  But…arithmetically, that does not translate to half of bipolar parents having a bipolar child!  You can calculate the actual arithmetic result and play with input assumptions using this calculator. (Note:  “Bipolar parents” means either 1 or 2 bipolar people. This doesn’t distinguish the evidently higher risk from having 2 bipolar parents.)

Assumptions: 

A __3__ % of individuals are diagnosed with bipolar disorder  (30/1000)

B: __50_ % of diagnosed bipolar individuals have one or two bipolar parents (15/30)

[Interactive calculator forthcoming, where you can change assumptions]

Calculated result:

C: __25_ % probability that a bipolar person’s child will have bipolar disorder (15/59)

D: __2__% probability that a non-bipolar couple’s child will have bipolar disorder (15/941). Note that D < A, because a genetic linkage means that prevalence across all individuals (A) is higher than prevalence only among offspring of non-bipolar parents.

E: _8.5__x higher likelihood that a bipolar person’s child will have bipolar disorder (C/A), compared to if the disease were randomly distributed with no genetic link

  Child  
  Bipolar not Bipolar  
Parents Bipolar 15 44 59
not Bipolar 15  926 941
    30 970 1000

Personal context:  In 2002, I married a man who was subsequently diagnosed with bipolar disorder.  The diagnosis (and second through seventh opinions to confirm it) was a comfort, as he had “always felt like there was screw loose”.  He was, understandably, terrified of passing on the disease.  In 2007, his immigrant parents wrote him off as a “shame to the family” (for the diagnosis) and “addicted to drugs” (for taking prescription anti-psychotics).  My husband’s mother wasn’t diagnosed, but he felt she had some type of mood disorder; also, his maternal grandfather, who died back in the old country, was described suggestively as an eccentric who had gambled the family fortune away.

So, we planned to adopt a child… as soon as we recovered from being financially wiped out from keeping my unraveling husband alive and functioning in a pre-ACA medical system.  Even our 401ks had to be liquidated to pay for treatment.  I quit my career to manage our chaotic life.  Then, the whole economy collapsed, adding insult to injury.  My conservative parents wrote me off for having “chosen” to make the proverbial marriage bed I was laying in.  Environmental stress skyrocketed; his symptoms got worse.

One spring day in 2011, my husband went off his meds and vanished.  In the disorienting aftermath, I tried to have my eggs frozen, so that no matter what happened in the uncertain future, I’d at least retain the choice to have a biological child. 

The hospital ethics board deliberated as to whether I should be allowed to undergo the surgical procedure, given the possibility that my missing bipolar husband might return and I might have him fertilize the eggs.  But, they realized that refusing me the procedure would constitute a violation of my rights – not to mention that it smacked offensively of eugenics.  So I got the go-ahead.  Nonetheless, the procedure never happened.

The missing bipolar husband resurfaced in a violent manner.  A trial was set for the same day as the procedure.  Both proved to be strictly immovable due to judicial machinations and ovulation timing.  (It wasn’t until a year later that a mounting list adverse coincidences made it no longer paranoid to suggest this scheduling overlap hadn’t been a coincidence.)  My main witness was my semi-estranged mother.  She threatened to sabotage the trial if I had her appear without me there in person.  My second witness was an acquaintance who wouldn’t guarantee he’d take time off work to show up.  (Crossing the brown line of interracial marriage meant certain people refused me sympathy or protection, seeing my suffering as a platform to share essentialist beliefs about how “those people”  act; long-suppressed bias surfaced as relatives suggested I leave a man who had in fact already left me.)

Thus, I faced a Sophie’s choice between my near-term safety and a hypothetical future child.  Fear drove me to the courtroom instead of the hospital.  By the time the doctor reset my reproductive cycle to reschedule the procedure 4 months later, my late-30s ovaries had abruptly and irrevocably slipped off the cliff of fertility.  And, as of this writing 6 years later in 2017, the long shadow of my beloved husband’s mysterious disappearance and violent return are such that I still don’t have a stable job or a new husband, and thus am not qualified to adopt, either.

Today, people callously tell me to be “glad” I don’t have a child — glad because the child could have been bipolar if my husband had come back, and glad because when he didn’t come back I might have opted for the tough road of single motherhood if I had had the eggs.  But choice is a transformative thing that shouldn’t be so flippantly discounted.  Choice makes the difference between sex and rape, between car-camping and homelessness, between sharecropping and slavery, between euthanasia and homicide, and between a couple electing to be child-free and…. a tragedy for which our culture doesn’t even have a word:  a woman being denied the possibility of motherhood due to the actions of others.

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