The Lakes Plain languages were tentatively grouped by Stephen Wurm with the Tor languages in his Trans–New Guinea proposal. Clouse rejected this connection to the Tor languages and grouped them with the Geelvink Bay languages. Malcolm Ross classifies the languages as an independent family, a position confirmed by Timothy Usher. Because of the apparent phonological similarities and sharing of stable basic words such as ‘louse’, William A. Foley speculates the potential likelihood of a distant relationship shared between the Skou and Lakes Plain families, but no formal proposals linking the two families have been made due to insufficient evidence. Additionally according to Foley, based on some lexical and phonological similarities, the Keuw language may also possibly share a deep relationship with the Lakes Plain languages. Like the East Cenderawasih Bay, Trans-New Guinea, and South Bougainville language families, Lakes Plain languages have ergativecase marking systems. In contrast, most languages of northern Papua New Guinea have accusative case marking systems.
Clouse (1997)
Clouse internally classifies the Lakes Plain family as: ;Lakes Plain superstock
Clouse concludes that the East Geelvink Bay languages are the most closely related to the Lakes Plain languages, forming a wider Geelvink Bay phylum with it.
Usher (2018)
The Lakes Plain languages as classified by Usher are as follows: Not included in the above classification, Kaiy, Kwerisa, Doutai and Waritai are presumably also Central Lakes Plain; the same for Obokuitai and Biritai. Clouse had placed them closest to Papasena and to Eritai, respectively, and they might form dialect clusters with those languages. There are particular questions about the inclusion of Saponi, Kehu and Tause.
Pronouns
The pronouns Ross reconstructs for proto-Tariku are, The corresponding "I" and "thou" pronouns are proto–East Lake Plain *a, *do, Awera yai, nai, and Rasawa e-, de-. Saponi shares no pronouns with the Lakes Plain family; indeed its pronouns mamire "I, we" and ba "thou" are remenincent of proto–East Bird's Head*meme "we" and *ba "thou". However, Saponi shares half its basic lexical vocabulary with Rasawa, and Ross left it in the Lakes Plain family pending further investigation. The Tause language was also previously grouped amongst the Tariku group of Lakes Plain languages. Ross transferred it to the East Bird's Head – Sentani languages on the basis of pronoun similarities in hopes that this would promote further research. Below are pronouns in selected Lakes Plain languages as given by Foley :
Phonology
Lakes Plain languages have remarkably small phonemic inventories, rivaling even those of Polynesian languages.
Consonants
Clouse and Clouse note many of the Lakes Plain languages share several unusual phonological features. While Papuan languages typically have at least two nasal phonemes, this is not the case for Lakes Plain languages. Although phonetic nasals do exist in most Lakes Plain languages, they do not contrast with the corresponding voiced stops. Doutai, Sikaritai, Obokuitai and Foau lack even phonetic nasals. Additionally, no Lakes Plain language has a liquid phoneme. Clouse reconstructs a typologically remarkable consonant inventory for the ancestor of Lakes Plain, consisting entirely of only five stops:
This results in Lakes Plain languages having high functional load, meaning that there are many words with small distinctions in sounds.
Vowels
Many of the languages have very high constricted vowels; in Doutai and Kirikiri these constitute separate phonemes from /i/ and /u/. The fricativised vowels seem to have developed from deletion of a following consonant. Clouse reconstructs a five-vowel system for proto-Lakes Plain, not unlike Japanese or Spanish:
Front
Back
High
*i
*u
Mid
*e
*o
Low
*a
Tone
Lakes Plain languages are all tonal. Clouse and Clouse reconstruct tone in proto-Lakes Plain. Duvle and Sikaritai have only two tones, high and low, but all other Lakes Plain languages have more than two tones. All West Tariku languages have both rising and falling tones. Iau is the most tonally complex Lakes Plain language. Unlike other Lakes Plain languages which can be disyllabic or trisyllabic, Iau word structure is predominantly monosyllabic. Iau has eight phonemic tones, transcribed by Bateman using numerical Chao tones : high, mid, high-rising, low-rising, high-to-low-falling, high-to-mid-falling, mid-to-low-falling, and falling-rising. Four of the eight Iau tones occur on short vowels, while the remaining four occur on long vowels and often correspond to disyllabic words in other Lakes Plain languages.
Morphology
Unlike most Papuan languages to the east, words in Lakes Plain languages do not have gender or inflection. Bauzi, an East Geelvink Bay language spoken to the northwest of the Lakes Plain family, also does not have gender. As analytic isolating languages, there is not much morphology.
Basic vocabulary
Basic vocabulary of the Lakes Plain languages listed in Foley :
Proto-language
Clouse reconstructs basic vocabulary for proto-Lakes Plain and other lower branches. ;Lakes Plain reconstructions by Clouse
