10 February 2016 Last updated at 14:11 GMT
A team at the University of Sussex put heart monitors on horses and showed them pictures of faces to see how they reacted to different emotions.
They found that the angry faces made the horses' heart rates speed up quicker than the happy faces.
Amy Smith, one of the researchers, explains more.

Such investments should be registered in the same way as securities sales, according to the Securities and Exchange Commission (SEC).
It follows a rise in the number of Initial Coin Offerings, or ICOs, through which crypto-currency start-ups have sought quick financial backing.
However, there are concerns that ICOs are creating an investment bubble.
More than $1bn has been raised so far this year in such transactions but there are questions over how investors are to be protected should a venture backed with digital coins fail.
Now the SEC has said that issuers of "distributed ledger or blockchain technology-based securities" must register offers and sales.
"The innovative technology behind these virtual transactions does not exempt securities offerings and trading platforms from the regulatory framework designed to protect investors and the integrity of the markets," said Stephanie Avakian, the SEC enforcement division's co-director.
It was sometimes difficult to see how a firm had justified a large evaluation for its ICO, said Eitan Jankelewitz, a technology specialist and partner at law firm Sheridans.
"If you consider the size of the investment compared to the type of investment that a professional would make, they look disproportionally large," he told the BBC.
He also pointed out that regulators don't need to rely on legislation specifically dealing with crypto-currencies to enforce rules on securities and investments.
"Those sorts of regulations tend to be more principles-based," he explained.
"If a new technology comes along that allows a new activity to be undertaken, then the law would still apply."
The SEC made its comments on ICOs in a report on a crypto-currency investment fund known as the DAO.
Digital coins worth $60m ($41m) at the time were stolen after the DAO was hacked last year.
While the SEC has not decided to make civil charges as a result of its investigation into what happened, it is using the case to caution industry and the market.

The London-born, USA-raised 21-year-old will count as a foreign import because of his overseas junior development.
"My dad loves it," he said. "It's different from soccer, but he loves the intensity of it.
"He's only an hour and a half away, so he'll be able to get to watch me play more than he's ever done."
Asked whether his football-mad dad, a former Brentford trialist and passionate Celtic and Scotland fan, understands the game, Stewart added: "I've told him that, whenever we score, or the opposition score, he's just got to make sure he cheers for the right team."
"Liam is a very important signing for us," said Blaze head coach Danny Stewart. "He adds to our depth which will be crucial next season."
Blaze, who won the Elite League five times in eight seasons between 2003 and 2010, finished sixth last season.
Liam Stewart first developed an interest in ice hockey after moving to America to live with his mother in Southern California at the age of three.
He played at a junior level with the Spokane Chiefs, before making two play-off appearances for the Quad City Mallards, and then making 13 appearances last season for the Alaska Aces.
"I was there for a couple of months before I got injured," he said. "The injury is fully recovered now and I am itching to get back on the ice to start the season."
Horses can tell if people are happy or angry, according to a new experiment.
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A US regulator has said firms looking for backing via crypto-currencies should be more strictly regulated.
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Elite League side Coventry Blaze have signed Liam Stewart, son of music legend Rod and former model Rachel Hunter, for the 2016-17 season.