New York, Nov 26: Even a 10-month-old infant can tell how badly you want something by observing how hard you work to achieve it, says new study that suggests that babies learn to infer motivations of others much earlier than previously thought.

The ability to assess how much someone values a particular goal requires integrating information about both the costs of obtaining a goal and the benefit gained by the person seeking it.

The study published online in the journal Science also suggests that babies acquire very early an intuition about how people make decisions.

"This study is an important step in trying to understand the roots of common-sense understanding of other people's actions," said study co-author Josh Tenenbaum, Professor at Massachusetts Institute of Technology (MIT) in the US.

"It shows quite strikingly that in some sense, the basic math that is at the heart of how economists think about rational choice is very intuitive to babies who don't know math, don't speak, and can barely understand a few words," Tenenbaum said.

Previous research had shown that adults and older children can infer someone's motivations by observing how much effort that person exerts toward obtaining a goal.

The new study wanted to learn more about how and when this ability develops.

In the experiment, the researchers showed 10-month-old infants animated videos in which an "agent," a cartoon character shaped like a bouncing ball, tries to reach a certain goal (another cartoon character).

In one of the videos, the agent has to leap over walls of varying height to reach the goal.

First, the babies saw the agent jump over a low wall and then refuse to jump over a medium-height wall.

Next, the agent jumped over the medium-height wall to reach a different goal, but refused to jump over a high wall to reach that goal.

The babies were then shown a scene in which the agent could choose between the two goals, with no obstacles in the way.

An adult or older child would assume the agent would choose the second goal, because the agent had worked harder to reach that goal in the video seen earlier.

The researchers found that 10-month-olds also reached this conclusion.

When the agent was shown choosing the first goal, infants looked at the scene longer, indicating that they were surprised by that outcome.

Length of looking time is commonly used to measure surprise in studies of infants.

"Across our experiments, we found that babies looked longer when the agent chose the thing it had exerted less effort for, showing that they infer the amount of value that agents place on goals from the amount of effort that they take toward these goals," lead author of the study Shari Liu, a graduate student at Harvard University, said. (IANS)