Why does running water make me need the toilet? © Getty Images

Why does running water make me need the toilet?

We’ve teamed up with the folks behind BBC World Service’s CrowdScience to answer your questions on one topic - this week it's all about how running water causes the urge to pee.

Generations of parents will tell you about the running tap trick. Whether you’re potty-training a toddler or trying to get a young kid to pee before bedtime, the trickling sound from a tap will help them.

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In hospital contexts too, urology nurses will use the sound of water to help patients who, as part of an examination, are in the awkward situation of needing to relieve themselves on demand in front of others. Recently, researchers even tested a smartphone app that played watery sounds to help urology patients pee.

Despite the familiarity of the trickle-sound technique, it remains far from fully understood. The most obvious theory for why it works is via a process of conditioning. The sound of a trickle of water, especially into a pool below, matches closely the sound of urinating into the bowl of a flush toilet. For many of us, the act of peeing has been associated with this sound multiple times a day, for years! The idea, then, is that hearing the trickle somehow triggers a conditioned reflex to pee.

However, this explanation can’t account for the fact that people who’ve never used a flush toilet – as is the case with billions around the world – still reportedly experience the effect.

An alternative theory is that the trickle sound makes us feel calm and safe, and so facilitates the activity of the ‘parasympathetic nervous system’. This system acts to dampen down the ‘sympathetic nervous system’ that’s responsible for the fight-or-flight response. The result is a physiological state that relaxes the muscles that control the bladder, making us feel the urge to go.

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