The research took place in Aberystwyth

But the report said that the ability to speak Welsh may not be the only reason for the difference in earnings.

Professor Henley also suggested that Welsh speakers may be better 'networked' and better informed about new job opportunities.

Other explanations offered for the differences stem from the Welsh Language Act of 1993 and bilingual children having improved cognitive and creative abilities.

Financial advantage

Working on data collected in the 1999 Welsh Household Panel Survey, the group were aiming to discover if there is a financial advantage to to having more than one language in a bilingual economy.

They may be better informed about labour market opportunities as they arise

Professor Henley, University of Wales, Aberystwyth

However, the use of Welsh in work is not the only reason behind this difference in earnings.

Professor Henley explained that, while bilinguals earn on average 60-75 pence an hour more than monolinguals, the use of Welsh at work accounted for a difference of only 36 pence an hour between the two.

He believes the study shows that there are three possible reasons why the ability to speak Welsh appears to give a comparative advantage in the labour market.

Better networked

The first suggestion is that Welsh speakers are better 'networked'.

"They may be both better informed about labour market opportunities as they arise, and also enjoy an adverse selection effect operating in their favour by being better known to potential employers," he said.

The second reason given by Professor Henley is that the 1993 Welsh Language Act requires public bodies to have a Welsh language policy and therefore employ Welsh speakers.

Finally, he felt that the difference may reflect that bilingual children have improved cognitive and creative abilities, and are rewarded accordingly.

Not enough

Professor Henley also concludes that, given the type of recognised qualification that bilinguals have, they should earn more.

"The evidence shows that those who speak both languages should earn at least �1 per hour more than their monolingual counterparts," he said.

In Wales, approximately 20% of the population speak Welsh, while estimates suggest that between 50 and 66% of the world's population are bilingual.