In the study, people who watched videos of a man and a woman sharing a meal were more likely to rate the pair as "highly involved" if the one member of the couple fed the other food they'd already nibbled on, compared with pairs who fed each food they hadn't contaminated with their saliva.
"Sharing may communicate special significance if the shared food is contaminated with the germs of the other person, as when sharing a spoon or eating off of the same small food item," Thomas Alley, the study author from Clemson University in South Carolina, said.
According to the study, although the behaviour is not well understood, animals of many species share contaminated food with each other, and in people, there is some evidence that sharing food contaminated with saliva might offer protection against some viruses.
118 college students watched videos, each less than a minute long, of a man and a woman at a meal.
In some videos, the man fed the woman food he had taken a bite of. In other videos, he fed her food he hadn't nibbled. Other videos likewise showed the woman feeding the man, and in some videos, neither fed the other.
The students were asked to rate how well the pair knew each other, how much each member of the pair was attracted to the other, and whether the pair would grow closer or more distant over time.
"Videos featuring contaminated feeding consistently produced higher ratings on "involvement" than those showing uncontaminated feeding, which, in turn, consistently produced higher ratings on involvement than those showing no feeding behaviours," Alley said.
This effect was seen regardless of which member of the pair did the feeding.
The participants also rated the couples as more attracted to each other when contaminated food was shared.
The results of the study also indicated that when a video showed a man feeding a woman, he was seen as more attracted to her, compared with videos showing a woman feeding a man.
This may have been due to participants' views of men as resource providers, and women in general are more nurturing, according to the study.
"Cases of 'contaminated' food sharing have some similarity to mouth-to-mouth kissing in that both reflect a willingness to accept biological 'contamination' from the other person," he added.
The study has been published online in the journal Appetite.