If you're the sort of gentleman who holds the door open for a lady – or the sort of woman who expects him to – then be warned.

Such acts of chivalry may actually be 'benevolent sexism' in disguise, according to researchers.

Experts say this type of sexism is harder to spot than the 'hostile sexism' we are more familiar with – because it often masquerades as gallantry. It is typified by paternal and protective behaviour, from encouraging smiles to holding doors open.

Sexist? Boston researchers carried out a study into types of sexism in men. They say there are two types of sexists, identifiable by their smile (stock image shown)

'Hostile sexist men' were less approachable and less friendly while those who displayed 'benevolent sexism' were nicer and smiled more

US researchers argue that while women may enjoy being showered with attention, benevolent sexism is 'insidious' and men who are guilty of it see women as incompetent beings who require their 'cherished protection'.

Professor Judith Hall, of Northeastern University in Boston, said: 'Benevolent sexism is like a wolf in sheep's clothing that perpetuates support for gender inequality among women.

'These supposed gestures of good faith may entice women to accept the status quo in society because sexism literally looks welcoming, appealing and harmless.'

Working with a male colleague, Professor Hall paired 27 men with women they had never met before and gave them a quiz game to play and time to chat afterwards.

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The men, who were aged 18 to 22, also filled in a survey designed to detect sexist attitudes.

Examples of hostile sexism included agreeing that women tend to exaggerate problems at work, while someone who is benevolently sexist will believe women should be rescued before men from a sinking ship.

They found men whose answers led to them being classed as benevolently sexist smiled more while playing the quiz game and chatting.

They were more patient while waiting for their female partners to answer the trivia questions and warmer, friendlier and chattier than those who were hostile sexists.

Writing in the journal Sex Roles, the researchers said that while such behaviour may be well-intentioned, this doesn't mean it isn't harmful.

Men who are guilty of benevolent sexism see women as 'warm and pure, yet helpless and incompetent beings'.

Study co-author Jin Goh said: 'Benevolent sexist men hold women in high regard and are willing to sacrifice themselves to save and protect women.

However, benevolent sexist men perceive women as the weaker gender at the same time.

Thousands marched on 8 March 2015 for International Women's day. More than 1,000 women and men were marching for gender equality in New York City today - pictured here - as millions of people across the world rallied to demand change

'Unless sexism is understood as having both hostile and benevolent properties, the insidious nature of benevolent sexism will continue to be one of the driving forces behind gender inequality in our society.'

But critics have argued that there is nothing wrong with men trying to protect women as they are usually the physically stronger sex.