How reliable is MyHeritage when it comes to first cousins

[AInightingale]

I haven't been a member long (since NY) but the site has flagged up a first cousin match for me. It's a bloke I've never heard of and his surname is also an unknown.
MyHeritage has been v accurate in identifying a first cousin once removed and a second cousin of mine, so I'm a bit bamboozled by this. His tree is private. Do I open a can of worms by messaging him? Or might he just be a second or even a third cousin I don't know about? It's a 10.4% DNA share with 734 cM, largest segment being 98cM. My father is adopted so that's the strongest possibility obviously, but it's not 100% certain that it's his side of the family. The adoption thing is also sensitive but there was talk that he did have a brother.

Can you click on to see if there are any shared matches? This might tell you what side of the family it is against known matches.

DNA painter tool suggests any of these relationships:

82% Great-GrandparentGreat-Aunt / UncleHalf Aunt / Uncle 1CHalf Niece / NephewGreat-Niece / NephewGreat-Grandchild

18% Great-Great-Aunt / Uncle †Great-Great-Niece / Nephew †Half Great-Aunt / Uncle †Half Great-Niece / Nephew †Half 1C 1C1R

At the amount of matching cm it is pretty accurate.

Thanks for reply. No, he is not coming up as sharing DNA with my maternal 1st OR and 2nd cousins, which discounts the possibility that he's some unknown child of my mum's two brothers, as the DNA link would be there via my grandmother. The other possibility is that my maternal granddad had a child at some point with another woman, likely from an affair, and this guy is their son. What a can of worms that would be.

It is fascinating how DNA can create as well as solve mysteries.
Are you going to contact him?

DH's grandad ran off with his SIL and had a load more children. It plays havoc with the shared DNA proportions of his cousins.

If your father is adopted then I would start from there.

Depending on the age of your father siblings (even twins) were often separated. Being a single parent is not easy but in the past it was even more difficult, sometimes a child would be kept by the mother but a second one would not be.

If you want something interesting to watch the documentary 'Three identical strangers' or the Mr Ballen version are worth a watch.

Personally I would message saying you have matched as a first cousin and that your dad is adopted and can he shed any light on things?

Personally I would message saying you have matched as a first cousin and that your dad is adopted and can he shed any light on things?

This.

At this point you both know about the existence of the other. You may as well break the ice and ask the question. You don't have anything to lose by him not replying.

Yes it's a can of worms but that's the nature of DNA tests. Both parties ultimately have to understand this when they take them. And I do think it's partly why you do.

My Dad and my MILs family have both returned odd results. My Dad was expecting it but it wasn't where he thought it would be. And we expected it with my FIL and Mum and nothing (so far).

Is your father still alive?

Thanks for replies. No my dad passed some years ago, he knew he was adopted but never wanted to know details of his background and we respected that, I would not pursue this if he was still alive. Any siblings would be aged 80s/90s now. This guy is middle-aged.
I sent a vague message introducing myself and mentioning that we are aware of an adoption - he's obviously received the same notification as me so he's got a surprise too, he may not be aware that his parent had an adopted sibling so will have to tread carefully.
I don't think it was anything to do with my grandfather as that would mean a half first cousin and the shared DNA % seems a bit too high for that. Relieved if that's the case, wouldn't like to think of that situation.
He may never reply, which would be disappointing, but I'll update the thread if he does.

@AInightingaleI can help you with this even if he doesn’t reply as I’m a professional genealogist and DNA detective. This is right up my street, so to speak. Drop me a PM if you’re interested and I will assess your matches and let you know what I think