If someone is given a lottery ticket is there an unspoken agreement about what happens if it is a winner?

It is entirely arbitrary. An agreement can be made in advance, but usually a ticket given as a gift, the winnings then become the gift. The winner might share out of the goodness of their heart.
In a recent legal case, a worker took tickets purchased by the group, plus bought one for herself. When one ticket won big, she tried to claim it was the one she bought personally, but a judge didn’t agree. He ruled the winnings must be shared.

Giving a lottery ticket or accepting one is a recipe for disaster. It is a situation better avoided. The chances are the ticket won’t win but if it wins big a whole Pandora’s Box of evils will be unleashed with the potential to destroy relationships and long-standing friendships. In your situation I would buy a ticket or two and give them to someone else before the draw takes place.

I don’t think there’s an unspoken rule, but for me, if someone gave me a lottery ticket and it won big (a million or more), I’d try to throw 10% to the giver. If it won a thousand, I might take them out to dinner.

In the situation like @YARNLADY is talking about, when we buy lottery tickets for a group at work, we always make xerox copies of the tickets, so it’s clear to all what the winning numbers are. If the person who did the purchasing bought extra for themselves, there would be no misunderstanding what tickets belong to the group and what tickets belong to the individual.

I would totally go by gut feeling.
If I win €50000 I wouldn’t give anything (other than a small amount so they can buy something to treat themselves) and book a flight to Greece and use all of it to buy a small house.

We’re I to win a million, or 10, then I can imagine I’d split it in equal parts and give my family members all a part and keep the rest (and again go to Greece, and hand out some there too, to family and friends).

In the late 1980s I would get a ticket or two from a co-worker that lived in Florida mailed to me (I lived in New England). We both worked for the same company and had common projects we worked on. If I won I would have to send the ticket back to him to cash it in. Deal was he could keep a part of the winnings !

@jca2 I, too, was thinking I’d give the giver 10% if it were big.

I gave tickets to a friend last night, partially as a joke and partially to cover the cost of the meal he prepared – which was quite expensive. He was the one who brought up the subject. Before scratching he said he would split winnings 50–50. I said that was too much – I’ll take 25%.
He won $8. I told him to keep it. :-)

No. If you give the ticket to someone and it wins, they win. If you don’t like that arrangement don’t give tickets away without a signed agreement.