THUMB sucking can be a hard habit to kick, as Marlene Headington, a psychologist specialising in addiction, knows. She has been sucking hers for 37 years. “It feels comforting,” she says.

She reserves her habit for bed, although her thumb still sneaks into her mouth when she is tired or stressed. Her teeth are fine and she struggles to sleep without it, so she has no plans to stop.

“I don’t see the point.”

Thumb sucking is “like having an addiction to smoking or drugs, it’s very difficult [to give up],” says speech pathologist Andreanne Blanchard from Sydney Spot, who often sees families struggling with a child’s thumb habit.

media_camera Marlene Headington said her 18-month-old son Sonny uses a dummy. Picture: Toby Zerna

It is a natural response to a newborn’s sucking reflex — some begin in the womb. Many kids grow out of it. But some cling to the habit well beyond babyhood as a way to calm themselves when they’re tired, upset or stressed.

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Yet it can cause dental deformities, ranging from buck and crooked teeth to an under-sized top jaw. A thumb-thwarting industry has sprung up, ranging from products that “break the pleasure of the suck” to services such as thumb clinics and hypnosis.

Nevertheless, the number of people who kept sucking their thumb into adulthood seems a sizeable, if secretive group.

High-profile members include Bachelorette star Sophie Monk and pop star Rihanna.

media_camera Bachelorette Sophie Monk has admitted to thumb sucking as an adult. media_camera Singer Rihanna admitted to sucking her thumb past childhood. Picture: AP Photo/Mamadou Diop

Even Oscar-winning actress Emma Stone sucked her thumb until she was 11 and admits to occasionally still finding comfort in it, despite seven years of invasive orthodontic work to fix the damage her habit caused.

Embarrassment often forces teenagers to stop. One mother told The Sunday Telegraph her 17-year-old daughter finally gave up her thumb and her snuggly toy at age 17, when she had her first boyfriend.

Ms Headington, now a mother herself, sucked her thumb “anywhere and everywhere” until she was in her late teens. She had often been warned about dental problems but never experienced any.

When she began feeling awkward about it, she “set a limit to myself — I would just do it at home, so that’s what I do now. It’s so comforting and calming”.

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Her grandfather, stepdad and husband have all encouraged her to give up, but she has never made a serious attempt.

“I think, knowing the comfort it brings me, and it’s not causing any harm, if it’s not causing issues with teeth, to have that constant comfort is lovely,” she said.

Even if she did want to give up, she is not sure she could.

“When I have tried not to, when I stayed over at someone’s house, I can’t sleep,” she says.

“I will wake up with my thumb in my mouth anyway.”

Her son, now 18 months, uses a dummy, although she wouldn’t be worried if he sucked his thumb.

And as a psychologist and addiction specialist as well as a committed thumb sucker, Ms Headington urges parents to use positive enforcement rather than negativity to encourage their children to give up. If they are negative, “it becomes something that’s forbidden and shameful”.