The Nasioi language is most closely related to Nagovisi (Sibbe) and forms, with Siwai (Motuna) and Buin (Telei), the south Bougainville Papuan stock. According to Allen and Hurd’s  survey, there were more speakers of Nasioi (with its dialects) than any other language in the Bougainville District. These people occupied a variety of ecological niches. They spanned the coastal areas through fertile valleys and up into the high hills. This meant that there was some variation in subsistence patterns. Those living in the valleys had access to more and different products, and could act as middlemen in exchanges between coast and hills. Hill dwellers in what is now called Kongara could not raise coconuts or sago, and so were dependent on their fellows residing at lower altitudes for these items. Villagers on the coast not only had more contact with Austronesian speakers, who reportedly taught them pottery making, but also had access to all the products of the sea. Some Nasioi had more contact with Buin speakers, others with Nagovisi, still others with Austronesian speakers both coastal and inland, specifically Banoni. Exchanges and occasional intermarriage thus took place across both ecological and linguistic boundaries.
Despite these environmentally conditioned differences, Nasioi possessed a relatively uniform culture which can be seen as one variant of a south Bougainville Papuan pattern. Their settlements, whether coastal or inland, were small, with often no more than a few households. People lived in houses raised on posts, usually occupied by husband, wife and children. There seems never to have been any real shortage of land for subsistence. People moved freely, whether to develop new gardens, to avoid disputes with others, or to flee an area that had developed a reputation for sorcery or other supernatural malaise.
One aspect of social organisation was most stable: every Nasioi belonged to a named matrilineal descent group, usually glossed by anthropologists as a clan. Not all members of a clan lived together but were dispersed throughout the entire Nasioi territory. Only those clan members who lived together cooperated on everyday tasks. Ideally, one should marry outside one’s clan. Clan membership was one principle through which important land rights were inherited. Bilateral cross-cousin marriage (marrying one’s father’s sister’s child who was also one’s mother’s brother’s child) produced long-lasting affinal relationships between two clans. Kongara informants in 1966–67 were emphatic in connecting that practice to other forms of balanced exchange, thereby keeping land and shell valuables within a limited span of kin and geography. Residence rules specified that a newlywed couple should set up housekeeping in the bride’s village.
All of these factors helped to create a society that was characterised more by equality than hierarchy. Women had status complementary, rather than subordinate, to that of men. Their role as gardeners, producing the bulk of village subsistence, was highly valued, as was their place in maintaining continuity of the clan. Maternal symbolism characterised Nasioi discourse; the epitome of any quality (like industry) was phrased as ‘the mother of (work)’. Social interaction was built around an ideal of balance. Thus, Nasioi contrasted their balanced exchange of food and valuables at marriage with the institution of bride price, of which they had heard from other groups. (One Nasioi even said ‘What we really did was exchange people’, a neat description of what anthropologists call bilateral cross-cousin marriage.)
Nasioi leaders, called oboring (pl. obontu), can thus be fairly described as ‘big men’, though compared to Siwai described by Oliver  they were rather small fry. Villagers described the important qualities of an oboring as those of generosity, industry, and knowledge. He was certainly supposed to give large feasts to establish and reinforce his status, but these were smaller in scale than elsewhere in the south, and the road to his status was open to others, not simply determined by heredity. He had to rally followers to amass the food for these feasts and, if his demands became too onerous, the followers would simply move away. (Though modern-day Nasioi may have overemphasised their peaceful nature, large-scale conflicts of the kind reported as having occurred in the New Guinea Highlands do seem to have been rare, as one might expect from the existence of adequate supply of garden land.) Another check on an oboring’s power lay in the fear that sorcery could be carried out as a leveling mechanism against an overweening individual. Fear of sorcery was generally a form of social control against all forms of transgression.
As noted below, by the time of my fieldwork missionisation had overlain earlier religious practices, but basic attitudes forming a world view showed continuity with the past. Most notable was a belief that all good things came from the spirits of the dead. It was these spirits who had to be propitiated with offerings of special food like pork, opossum or canarium almonds if children and pigs were to thrive, gardens to flourish and success to be achieved in hunting. As older Nasioi said ‘If you didn’t give them food, you would be the one to starve’. Ancestral spirits provided special abilities like healing to the living. Other beings with whom the living had to contend might be described as nature spirits or bush ogres, such as a fearsome water creature described as part eel, part crocodile, or hairy goblins with a taste for human flesh. Before missionisation, the dead were cremated on a funeral pyre.
Parkinson [1999: 212] said that a line could be drawn that separated headhunting in the south from cannibalism in the north. Although Nasioi in the 1960s would happily agree that this distinction held true for their northern neighbours, they did not discuss head-hunting as one of their own practices. However, they certainly spoke of a time when the dead were cremated and lower jawbones displayed in houses. It is not hard to imagine that such displays were sometimes of enemies slain in battle.