Interesting
I found this article, here is a portion from the link below.
http://www.regalis.com/adopt.htm
Before embarking on any project involving an adoptive ancestor or one born outside marriage, it is important that the researcher acknowledge and comprehend several factors. From 1860 until 1929, the Italian state (i.e. the Kingdom of Italy) did not recognize Catholic marriages. Although it is true that both ecclesiastical and civil marriages were performed for most spousal unions in certain parts of Italy (such as the Kingdom of the Two Sicilies) in the decades before 1860, the Kingdom of Italy refused to recognize ecclesiastical marriages altogether.
This means that an act of birth from, for example, 1875, might refer to a child as the son of "an unwed mother" who in fact was married in church but not at the town hall. Terms of one of the Lateran Treaties granted retroactive state recognition of these ecclesiastical marriages in 1929. Therefore, such acts must be confirmed by consultation of parochial records.
Hearsay may provide certain general information, but its legitimate research value usually correlates in some way (often inversely) to its social value. An unwed mother might have claimed, perhaps long after her child's birth and far away from the locality where she resided when she gave birth, that the natural father was a local aristocrat; families often perpetuated such stories to salvage the dignity of both mother and child. A genealogist, however, would require more than a merely circumstantial "allegation" of paternity in such a case-namely, a formal act of recognition sworn by the natural father before the vital statistics registrar.
An act of recognition would not have "legitimized" such a child for purposes such as succession to a title of nobility, though it may have allowed him, if only under very particular conditions, to inherit certain other paternal property, but in any event a married man would not likely consent to acknowledge publicly that he had committed adultery or fornication. The principle of legitimacy relates to crown (statutory) and church (canon) laws too complex to be described in detail here. Illegitimacy in Italy has never been as rigidly defined as in English law, which distinguishes, for example, between "bastards" and "adulterine bastards."