In the first instance, there were not many arguments for not assuming a single, pre-classical author for all thirteen plays. There was simply no ground for doubt (amongst others, Shâstri (1923-25), Pusalker (1968:22-23), Sarup (1927:118) and Warder (1974:262-333)). The structural similarities, and the plain language of the dramas provided enough proof to accept a single authorship. The presence of a play with the title Svapnavâsavadatta pointed to Bhâsa as this author, although Paranjape (1927:8) leaves it an open question whether Bhâsa is this author, or someone else.
The first who pointed to the structural similarities, the same plain language, and the use of the same archaic Prâkrit, were Shâstrî himself (1923-25:627-637) and Sukthankar (1923:167-187), who investigated the dramas one by one, and listed all recurring verses, pâdas, parts of verses, prose parts and resembling ideas.
The similarities mentioned by Shâstri (1923-25:636) are of a technical nature, and are as follows,
Most later scholars accepted Bhâsa as the author of the Trivandrum group mainly on the basis of one or more of these similarities. Amongst them are Keith (1923-25:295-299), Thomas (1925:104-107, and 1928:877-890), Sarup (1927:118), Lindenau (1918), Banerji Sastri (1921:367-382), Paranjape (1927:1-9), Devadhar (1926:29-64), Pusalker (1968), Printz (1921), Warder (1974) and, more recently, Bhat (1985).
In addition to the structural similarities and the similar plain language used in the dramas, the Prâkrit appears to be almost uniform in all thirteen plays (Banerji Sastri 1921:367-382). Printz (1921) investigates the Prâkrit of the plays in a philological way, and is convinced of its archaic character, which is the same in the thirteen plays. He further reaches the conclusion that this Prâkrit dates from the period after Ashvaghosha (before the 2nd century A.D.), but before Kâlidâsa (5th century) (1921:4). Thomas (1925:104-107) calls attention to the archaic use of the ending -âni for the accusative masculine plural, which is also found in the works of Ashvaghosha. In addition, the frequent use of the word âdhya - rich - in the Bhâsa dramas would be a strong argument in favour of a single author (1928:884). Keith (1923-25:296) goes even further, and explains the appearance of the same Prâkrit even in the later dramas on the basis of the popularity of the Bhâsa plays amongst the theatre companies of Kerala. The impact of Bhâsa would have been great, so the later playwrights would have imitated the Bhâsa Prâkrit.
Bhat (1985:64) disputes the supposition that the structure would be typical of the dramas of South India, as proposed by amongst others, A.K. and K.R. Pisharoti (see below).
Warder, finally, points to some corresponding themes, such as the fire theme (1974:320), the theme of the unfair blow with the axe (1974:309), the tragic theme (1974:312), the dying on the stage (1974:314), and the vows (1974:318). As to the dying theme, already Winternitz observed this point, adding that it is in contradiction to the rules as formulated in Bharata's Nâtyashâstra (Winternitz 1968:5). Dying on the stage is only seen in the "Bhâsa'' plays (Bhat 1985:48).