There are 11 million vacant houses in Europe, compared to 12.8 million vacant houses in Egypt alone. Square that with some 100 million homeless people around the world and 1.6 billion people lacking adequate housing, and the role of affordability and access in the housing crisis becomes clear.

But other factors contribute to keeping properties vacant – factors that are intangible, and have nothing to do with the physical characteristics of the house, but rather center around superstitions. While the percentage of homes that remain vacant due to superstition is hard to quantify, we take a look at some of the spooks that keep buyers away from certain houses.

Superstitious beliefs come in all forms and shapes. For some cultures, numbers are a big concern for homebuyers, such as cities in North America and Europe, for example, where thirteenth floors are often renamed. In others, particularly in parts of Asia, specialists may be consulted to assess how much good energy or chi (also: qi) a home will bring to its owners. For others still, murder, violent crime and suicide can make or break the sellability of a property.

“Murders don’t just affect the house in which the crime has been committed, but it also affects the entire neighborhood in which the house is located,” Ashraf Reyad, a Cairo-based broker who sells properties in high-end gated communities in the city’s newer districts tells progrss. He gives the example of a 2008 murder of the daughter of a famous Moroccan pop star and her friend in an affluent suburb – a high profile case that was said to be the result of an alleged theft attempt. Reyad explains that the area around the crime was stigmatized for a few years after the murder.

But in times of need, he stresses, cases like these can hardly account for the millions of houses remain vacant in cities like Cairo. “If crimes could stop people from buying houses, they might as well stop people from walking down the street,” he says.

In other cities, homicides bring a great deal of stigma to their properties. Take Dennis Nilsen’s house in London for example. Nilsen, 71, was a British serial killer and necrophiliac who strangled and drowned 15 men in his flat in Muswell Hill in North London, and then proceeded to dismember and dispose of them by flushing smaller body parts down the toilet and hiding their body parts around the house. Even though the murders were committed in Nilsen’s house in 1983, people still hold a deep-rooted grudge against the property, with some claiming that the property is valued at £60,000 ($79,057.5) less than comparable homes.

Lucky and unlucky numbers also have an impact on properties. In New York City, many high-rise condo buildings omit the thirteenth floor altogether because they don’t want to risk losing buyers who might think the number brings bad luck. Belief has it that superstition about the number 13 dates back to the Last Supper when Judas, the thirteenth disciple, betrayed Jesus Christ, who is believed to have been crucified on Friday the thirteenth.

According to the Otis Elevator Company, a common ploy in large office buildings is to leave the thirteenth floor unnumbered and fill the space with an elevator or air conditioning machinery. Also, almost 90 percent of high-rise buildings in New York do not have the number thirteen, according to a service manager for the Westinghouse Elevator Company.

Similarly, in China, the number four is considered unlucky because it sounds like the word “death,” whereas eight is considered lucky because it sounds like the word “wealth” – both of which play a role in the pricing and purchasability of properties. Chinese buyers often use the 3,000-year-old practice of Feng Shui to assess the energy or chi of a house, which dictates what a house should look like – inside out – to improve the luck, health and prosperity of its tenants.

But these associations carry much further than China. Chinese buyers in North America can be particular about specifications when it comes to buying properties – especially those buying luxury properties. According to a survey of Chinese-Americans by Better Homes and Gardens Real Estate and the Asian Real Estate Association of America, up to 79% of Chinese-Americans reported that they would be willing to pay more for a house if it was built to Feng Shui standards, and three-quarters reported that some Feng Shui no-go’s could be deal-breakers.

Further north in Calgary, Canada, the cultural beliefs of residents of Chinese origins are so key to their buying decisions that some brokers publish guidelines on what makes a house buyable to the local Chinese community.

While superstition isn’t about to make or break the housing market anytime soon (cities like Cairo, after all, house communities in cemeteries), some buyers still shy away from properties that they perceive as bad luck.