Kiddushin 73: What is the Status of That Offspring?

The Gemara continues to walk us through who can marry whom.  They speak about who is permitted to enter a congregation and they provide examples of people of different status entering different congregations as proof.  The Gemara notes that most men and women are permitted to marry and that we will see more cases of uncertainty than of cases where people are forbidden to enter congregations.  In addition, certain congregations will be made up of people who are of uncertain lineage.  We learn that when a person is a mamzer based on uncertain lineage, s/he is permitted to enter certain congregations.  This is used to prove that these people are permitted to marry Jews of 'unflawed' lineage.

The rabbis speak very briefly about when something is fixed, it is considered to be half and half.  In this case it refers to a woman who may have had intercourse with a number of different men within one fixed place, i.e. a city.  She may have married someone who disqualifies her from marrying a priest.  Thus this 'half and half' probability allows her to marry a priest and to stay married if this is learned ab initio. However, the rabbis are discussing the case of a shetuki, and a shetuki is not permitted to marry a Jew of unflawed lineage.

The Gemara moves to the topic of foundlings.  Are they deemed to be mamzerim?  Shetukim?  Unflawed?  Based on these past considerations of our rabbis, I would have assumed that significant proof of lineage would be necessary to assume a foundling has unflawed lineage.  However, that is not the case.  The rabbis share a number of different signs that the infant has been cared for: it's eyes had salve on them, its arms/legs were adjusted (it seems that babies were tightly bound in an attempt to straighten their limbs), it was wearing a note or an amulet, or if it was adorned with rings.  Any of these things deems the baby not subject to the halachot of a foundling.  

Further, the rabbis share a number of other signs that the baby has been cared for - it has not been left in an isolated setting, it has been left where it might be found, if it is a famine year (and then that is taken back - but the suggestion is that a famine might have forced parents of unflawed lineage to give up a baby).   There are a few witnesses who may testify to the baby's status, as well: a midwife, the foundling's parents, and one who exempts one's friends from uncertain impurity.  A midwife is a valid witness in this instance as long as she saw that the baby was not exchanged for a different baby during those early moments.  

Midwives are not usually deemed credible to testify, and they are also able to testify to whether babies born at the same time are kohanim, levites or israelites.  One objection, though, and she is not deemed credible.  This is compared to a purchaser and seller - the purchaser must have the item in his possession to be a credible witness regarding the sale.

Instead of assuming that people are of questionable or flawed lineages, the rabbis work to justify the assumption that a foundling's parents are of unflawed lineage.  Were people giving up their babies regularly? Were the rabbis concerned about the number of unflawed babies being born in a given place?  Were the Torah based directives lenient enough that the rabbis could justify their own leniency?  What else might have been going on?