Jealousy is is not a learned behavior, it comes with the package of being human.
See below link.
Babies start feeling jealousy at 3 months, study says
From the link
Lead author of the study, Prof. Maria Legerstee, professor of psychology at York University in Toronto, Canada, says even three months old baby may have ways of letting mom know by crying, kicking and turning in their seats when mom's attention turns to someone else. She says the behavior is quite normal and parents need not worry about it. It's a normal and appropriate reaction,' Legerstee said...The findings of the study will be included in next year's Handbook of Jealousy: Theories, Principles and Multidisciplinary Approaches.
"Before I bow down before any belief system, I look to see what axes the believer may have to grind. Prof. Legerstee has made a career in psychology studying jealousy. As you may note the referenced research will be published in a Handbook companion to another book on jealousy.
It seems that she has rather well developed jealousy in her shadow. It is little wonder that she finds it in babies and believes
The development of 'non-basic' emotions such as jealousy, pride, embarrassment and guilt are thought to develop during the second year of life, generally known as terrible two's, Prof. Legerstee said.
Please note 'non-basic emotions' that is the shadow 'develop' during the terrible twos, by parents trying to control the child's behavior by loading up the 'terrible bag' with dysfunctional self images of jealousy, pride, embarrassment and guilt. Note the relationship to the Seven Deadly Sins of Catholicism. None of which are worth much to the believer, but which are gold for the preacher or therapist.
The terrible two's are terrible because the child is learning to relate to others in herm society. This is a difficult process both for the child and the caregiver. The child will test behaviors and act out emotional reactions to peers and adults. It is all to easy for the caregiver to label the behavior as bad, even giving it a name, 'Don't be jealous of Billy you can play with Jane later.' Or worse 'Don't be jealous of Billy, there are others you can play with.' By the way while you are at it you can stuff that jealous self image in your shadow bag, it will be real useful when you are a teen.
It is harder but better for the care giver to find a socially acceptable way to help the child find a way into the Billy/Jane group. The jealousy is initially rejection by the group, which needs to be dealt with by finding ways to overcome the rejection. Rejection by the social group is an evolutionary fatal result. The two year old must learn to overcome the rejection.
The baby reacts to the rejection by herm most important social connection, herm mother, by crying, kicking and turning in their seats when mom's attention turns to someone else. One may impute jealousy, but abandonment by mom at 3 months is fatal.
If I may be so bold as to criticize the experimental protocol, I would ask if a normal mother would abandon attention to a 3 month old, for an animated emotional discussion with a stranger (to the baby.) Would not a normal mother have her hand on the stroller rocking it or otherwise showing the infant that hesh was in the social group?
(Don't get me started on abusive psychology experiments even at the University level)"
The experimental protocol was that mom was to bring a 3 month old baby to the lab, and mom would begin to talk to a stranger while ignoring the baby. While still ignoring the baby, the conversation would become animated with emotional content, laughing and presumably other emotional bonding signals. Mom is told to ignore the squirming, crying, kicking and other signals that the baby is feeling abandoned.
Now I was just an involved dad, and had the late afternoon baby sitting shift by choice, and would take the kids out in nice weather. I didn't have the same kind of mom-baby bond for obvious reasons, all I could do in the middle of the night was hang the baby on mom's teat, change the diaper when he was done and put him back in the crib. Yet I knew enough to keep a hand on the pram, and keep it moving when I stopped to talk with friends. The baby didn't have to cry and kick to get my attention if it lagged, just squirming around was the signal that my attention was not shared enough.
I cannot imagine a mom, even under lab conditions overcoming her natural instinct to attend to the baby at all times if only by rocking the pram. And I wonder what kind of issues went into the baby's shadow by abandonment even for a few minutes while the lab assistant noted the number of kicks, intensity of crying, and other indications that the baby was "jealous." I bet none of it was jealousy. But the "I can't trust mommy" separation anxiety got an early start. I would have fired our nanny for participating in that experiment. It was bad enough that mommy had to go to work. The nanny had strict rules on abandonment. And by the way strict rules on labeling behavior as well.