One of the small drawbacks of parenthood is the hearing loss that inevitable results from holding a crying baby in the vicinity of your ear for long periods. Parental deafness is a common but little discussed problem. And it explains why, if you're not a parent yourself, the couple at the table next to you in a restaurant can carry on a normal conversation even though their baby is emitting noises that make your ears bleed and cause dogs in the street outside to howl.
I've been researching this phenomenon for a possible compensation claim against the State; and my early findings suggest babies have a lot in common with mobile phones. You can't bring either of them to the theatre for one thing. But more importantly, both are held close to your ear regularly, and there is reason to believe both can fry your brain if you're not careful.
Admittedly, baby emissions are not the only explanation for brain-frying in parents, which is at least as likely to be the work of the infamous "parent hormone".
This is the naturally-occurring drug that causes you to exaggerate constantly about your child (I caught myself the other day telling someone how our five-month-old daughter was doing grade three piano lessons; in fact, she's only on grade two. I know - I'm ashamed of myself); and it also forces you to pick up a crying baby the moment it starts crying, rather than taking the sensible precaution of first putting some cotton wool in your ears.
To understand how parental hearing loss comes about, it is first necessary to consider how the ear works. As you will all remember from school biology, sound waves begin their journey to the brain by travelling down the "auditory canal", where they cause the eardrum or "tympanum" to vibrate.
The vibrations then pass through the three bones of the middle ear - the "hammer", the "anvil" and the "lucky horse shoe" - which refine them (checking for bad grammar, and so on), before passing them on to the inner ear.
Here, they travel along the "vestibular canal", transferring by barge to the "tympanic canal". The actual sound-sensing structure is the so-called "organ of corti," about which scientists have differed, and blah blah blah - as my daughter said when I tried to explain all this the other day.
The thing is, if there's one thing babies can't stand, it's bureaucracy. So when a baby has an urgent message to communicate to its parent, it simply refuses to queue at the inner-ear public counter, waiting for a customer service representative to become available.
Instead, it emits a particular pitch of sound-wave which is designed to blast a hole in the eardrum, and then charge straight through the door marked "Brain - Emergency access only," shoving security people aside and damaging sophisticated parental hearing equipment in the process.
And of course, for a baby, everything is an emergency. Which is why evolution has ensured that when it becomes necessary, even small babies can - for limited periods (usually not more that four hours at a stretch) make the same noise as aircraft.
It's a failsafe system, perfected over millions of years. A typical baby emergency occurs - a slight build-up of wind somewhere, or a sense that the parent's attention has wandered briefly to a subject other than baby welfare; then the baby screams; and then its parents - if they're in a restaurant - react immediately, asking each other to "pass the salt, dear".
Well, as you can see, evolution still has some work to do on this one. But there are other interesting aspects of parental deafness. For instance, curiously enough, it never extends to other people's children. Also, there is evidence that it affects the sexes differently.
Male parents have a particular difficulty with "n" words, especially where they occur alliteratively. A well-known example is the sentence: "That nappy needs changing, I think". Some men simply cannot hear this phrase, especially when there's football on.
But this legitimate medical condition is persistently misunderstood. Writing in The Irish Times letters page only last Tuesday, a Ms Gwen Woods of Dublin 4, blamed "nasal dysfunction" in males. More generally, an editorial in a recent issue of The Economist showed the extent of misunderstanding caused by the problem, when it suggested airline surcharges or even bans on babies, comparing crying children to smokers and mobile phone users.
In the resultant furore, one sympathetic reader suggested the compromise of a separate area on aircraft for members of what he called the "frequent crier club". Another reader asked "What about noisy drunks and middle-aged Texan ladies with nasal twangs talking about their operations on long-haul flights?" (common problems in international air travel, I think we all agree).
But my favourite response was that of "Jessica, aged 6" who pointed out that while nobody has to smoke or use mobile phones, everybody has to be a child once. Her letter continued: "You need children to pay for the pensions of miserable old people like you. Now pick on someone your own sise."
OK, I suspect that "sise" was a cunning ploy by Jessica's parents to disguise their involvement. Why, I bet even our daughter could spell the word "size," if we hadn't made a decision not to let her write until she's at least six months. But it was a nice thought, and The Economist had the good sense to give Jessica the last word on the subject.
Anyway, if you have any helpful suggestions about parental deafness, or how we can sue the State for it, why not give me a call? But make sure and shout, or I won't hear you.
Maeve Binchy is on leave