UKH

I have WhatsApp installed on my work phone for the sole purpose of following one chat about a junior sports team.

I deliberately kept this phone clear of personal contacts apart from close family, so how when I go to my contacts do I find about a hundred people including many of my partners work colleagues?

I have outlook installed with an account shared with my partner set up, could this be it?

How does WhatsApp discover contacts?

My main motivation for asking is that my daughter has just installed the app and her contacts list includes the parents of her school friends! She doesn’t even have an email account.

i know it works on phone numbers but where does it get them from?

Do you have a Facebook account ?

No, and not instagram either.

They have whatsapp installed on their phone and your number in their contacts. Whatsapp assumes that if they know you, then you (potentially) know them.

Probably its expanding the tree starting from the few contacts you have.   So if your wife is on WhatsApp and it has access to all her contacts that'll give it say a hundred new phone numbers and it knows your phone number so it'll check all her contacts that use WhatsApp looking to see if they have your phone number.  Once it gets a few accounts to start from and goes back a few levels it'll find a lot of the people you know.

Whether this is ethical is a different question.

It is pretty surprising what it's found and looking through the list (of several hundred) it seems to have harvested every contact I have on my email account.

It's clever but but it didn't ask me or explain what it was doing so certainly big privacy concerns there.

That doesn't explain most of the found contacts as due my usage of the phone I know that it's number is only known to about 5 people.

I'm  guessing it's your outlook I've got watsap but no email on my phone and don't have this, WhatsApps just got my phone contacts

It does ask you when you first install it. 

I have WhatsApp installed on my phone. I also have Facebook, Yahoo mail and Outlook. I didn't bother to take a huge amount of care when installing WhatsApp but it seems that I did click something to not let it harvest all the phone numbers in my contacts, as I have 527 phone contacts but nowhere near that many on WhatsApp. e.g. for the letter T, I have thirty phone contacts but only five on WhatsApp. This makes sense to be as WhatsApp surely will only be finding numbers attached to devices with WhatsApp installed, one would have thought. Not sure though! But your scenario sounds like you didn't opt out of something that I opted out of - also odd as I was not being "careful" (or whatever term you want to use) yet you seem to claim that you WERE. edit - not attacking you with that last bit! Just couldn't think of a softer way to phrase it.

OK I've worked out what's happened now - not Whatsapp being overly prying but Outlook oversharing!

Outlook syncs all contacts to my phone without asking (though i can turn this off), as I use an exclusive spam filter my contacts list is pretty big. Whatsapp just had a look at this big list and matched any contacts with whatsapp handles.

I did this; https://ascendix.com/blog/stop-sync-outlook-phone/

stopped whatsapp and restarted, gave it permission to see my contacts and bingo - only the people I want are in the list.

Of course i might want to turn it all back on again but it's good to know what is actually happening!

Good outcome then 

Why does your “exclusive spam filter” cause you to have a large contacts list? 

Only stuff from people on my contacts list can get into the inbox, the rest of the Nigerian financial proposals, performance enhancing pills and ladies who want me to look at them in lingerie goes straight to spam.