Too big a gesture, and you are seen as a dag. Too little, and you are considered rude, says a new anthropological study of Northern Territory drivers by Professor Adrian Peace.

''The protocol is pretty clear. There is a real danger of going over the top, and there is a real danger of doing too little,'' he said. While a grandiose wave was unsatisfactory, too small a movement of the finger from the steering wheel was dismissed. ''Their response would be, 'Why are you bothering?'''

Professor Peace started considering The Phatic Finger, the title of his new paper, while driving long distances as an anthropologist with the Northern Land Council. Phatic communication consists of verbal and non-verbal expressions, such as a nod or a ''g'day mate'', which are used socially and not usually for conveying detailed information.

What fascinated him was that the finger happened between strangers driving in places where breaking the 130km/h speed limit was considered ''as natural as breaking wind''.

Over time, Professor Peace discovered conventions about who, what, where and how the wave was done. He concluded the raising of the finger from the steering wheel or the slight wave of the hand was an expression of solidarity. While he warned against giving his research undue importance, these positive gestures far outweighed any incidence of road rage involving the finger turned the other way in disgust.

Sometimes people lifted a phatic finger to break the tedium. A van driver Professor Peace met in Katherine told him: ''Look, I do this trip twice a week, and it drives me cracked. Driving here and back is about as boring as anything you could think of. Mate, what else is there to do out there ?''

The phatic finger was used only far from urban centres and some people did not warrant acknowledgement. Professor Peace found his road trip companions acknowledged only working vehicles covered in ''grime, rust and mud'', ignoring rented vehicles or clean saloon cars.