Alicia Walker is doing something unusual: she's asking men - 3600 men to be precise - to send her photos of their genitals. All for the very noble purpose of scientific research.

Walker, an assistant professor of sociology at the Missouri State University, in the US, has put the call out for penis photos as she launched a study this month to explore how the size of a man's penis can affect the rest of his life.

"The kind of work I do is not for everybody," she said, quoted by USA Today.

In her study, the professor will look at how penis size, as well as the man's perception of that size, can have an effect in his health, sexual life, self image and even mental health.

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"So far I'm hearing a lot of anxiety and a lot of low self-esteem related to size," she said.

Walker hopes to receive at least 3600 photos of penises from men around the world. The participants in the study must be aged 22 or older and Walker will ask them to fill out an online survey and include the genitalia photos.

She made it clear that "these are not sexy pictures". "These are clinical pictures," she added.

The researcher says she doesn't want "anything dicey" so she is not recruiting locally. Instead, she's putting the call out through an online portal, hospitals and nightclubs.

She will include instructions for men to follow when measuring their flaccid and erect penis.

The researcher says the issue is an important one as a man's penis size can deeply affect his self-confidence and even trigger depression or body mysmorphic disorder.

"Some of them actually attempted suicide," she says of people she has studied before.