Jealousy in children and teenagers has been observed more often in those with low self-esteem and can evoke aggressive reactions. One such study suggested that developing intimate friends can be followed by emotional insecurity and loneliness in some children when those intimate friends interact with others. Jealousy is linked to aggression and low self-esteem. Research by Sybil Hart, PhD, at Texas Tech University indicates that children are capable of feeling and displaying jealousy at as young as six months. Infants showed signs of distress when their mothers focused their attention on a lifelike doll. This research could explain why children and infants show distress when a sibling is born, creating the foundation for sibling rivalry.