I seem to have ruffled some feathers with my blog on Bud Abbott. Instead of relying on gossip and oft-repeated stories, I looked at the evidence. Some people have a hard time with that because it challenges their long-held beliefs.
As far as Bud goes, the evidence points to his not being Jewish. Notice I did not say proves. More evidence could tilt the matter another way.
While we wait for that, let's move on to some sleuthing for Costello.
Lou's ancestry is usually given as mostly Italian with some mixture of Irish and French for the rest. They say his name was originally Cristillo and one story is that he changed it to Costello to match the surname of Dolores Costello.
Let's look at the census to see what it says. Remember, census info is evidence and not fact. We need to remember that because we get different info from each census!
In 1930, Lou's last name is Cristillo, his father is from Italy and his mother's parents are born in New Jersey.
In 1920, the last name is Cristillo, father from Italy and mother's parents born in New Jersey and England.
Here's a surprise in 1910: the last name is Costello! Where did that come from? Dad is Italian and mom has the same ancestry as 1920. Mom's dad (Louis Rege) is in the same household, born in New Jersey, half Italian and half French. It appears that his children had two different mothers, one born in England and one in France.
So far we're pretty sure that Lou's dad is 100% Italian but his mom's ancestry is rather murky. So far we have France, England, and New Jersey as possible origins.
In 1900, Lou's mother (Ellen Rege, line 54) is 17 and living with her parents who have been married for 18 years, so the hypothesis of a stepmother appears to be wrong. Her father has the same ancestry as in 1910 but her mother is from Pennsylvania with Irish ancestry. This is completely different from what is in other census records but it matches what it says in Lou's biography written by his daughter and also what we find in 1880 (Mary Tuohy, line 17).
We still don't know for sure where the name Costello came from (why did it pop up in 1910?) but we do have a good idea of Lou's ancestry. Doing the math, I come up with 62.5% Italian, 25% Irish, and 12.5% French. A long trip to find the facts, but worth the ride.