By the age of 11 he "was well and truly smitten with the history microbe" and had begun to keep notebooks recording the recollections of pioneer families around Gore and those of the surviving whalers and other old identities at Bluff and Riverton / Aparima, many of them descendants of marriages into the Māori community.
As a young man, Adam had formed a strong acquaintance with Māori of the Henley area, from whom he gained a detailed knowledge of traditions and ways of life.
Beattie was familiar with the archive-based regional histories of his time, such as Thomas Hocken's Contributions to the early history of New Zealand (Otago), and Robert McNab's Murihiku; but lacking academic training, and with opportunities for collecting first-hand accounts all about him, his writings were based upon interviews, buttressed with information from family notes, genealogies, and newspaper articles amongst more orthodox sources.
As this work began to bring him to the notice of professional ethnologists, Beattie sought ways of developing a career closer to his intellectual interests.
By 1919 his publishing success, and the acceptance of papers for the Transactions and Proceedings of the New Zealand Institute, persuaded Harry Skinner at the Otago University Museum to fund a year-long ethnological survey of southern Māori communities.
As a young man he had relied on his memory to write up conversations, but now he prepared written questions in advance, beginning with sixty-five and ending up with about a thousand.
Beattie had a working knowledge of Māori, but no facility in speaking it, so his interviews were conducted in English, though usually with a younger member of the informant's family to translate as needed.
Some clearly regarded him as offering the last chance to preserve substantial areas of traditional knowledge that they thought were not properly appreciated by their descendants.
His survey in 1920 was incomplete, however, because he gained little from people who had lived at Otakou and Kaiapoi and he lacked the time and resources to go far beyond Christchurch from his base at Weston, near Oamaru.
He ran that business until 1939, the earliest he could get an acceptable price for it after the depression, but remained in Waimate and, at the age of 59, was finally able to devote himself fully to writing and publication.
Beattie used the widest range of material and showed explicitly how his arguments and conclusions were revised as new evidence came to hand, but he did not discriminate between sources of varying quality.
With the recent acceptance attained by oral history and a resurgence of interest in the distinctiveness of southern Māori, Beattie's research has achieved a more general esteem.
In his collection of original material from oral sources in the South Island, he can be compared with Edward Shortland and Walter Mantell, but otherwise has no peer.