I'm sure Mendellian genetics apply. It becomes easier when a friendly one from a line of 20 friendly ones is bred to the same. I have birds that will walk away from food to come see me.
My roosters are very friendly and crave human attention, will eat out of your hand. They have a very low incidence of displaying human aggression. I can take a bird with no previous handling and perch it on my knee at a public event and it will let little children run up to it and pet it. I can...
I have had many different breeds through the years. I thought that Cochins were pretty laid back, had some fairly friendly RIR hens. None come even close to my Asils. They are like dogs with feathers, genuinely desire human companionship. Hens can go broody and raise the chicks on free range and...