Newborns may be considered ugly, new research suggests.
Adults find babies most appealing when they reach around six months old, a study found.
Previous research suggests babies have evolved "cute" characteristics, such as big eyes, chubby cheeks and cooing noises in order to bring out a nurturing instinct in adults that better ensures their survival.
Youngsters only typically take on these features at around six months old, which may be due to infant mortality previously being high; therefore delayed attachment may have made their deaths easier to cope with, past studies imply.
Lead author Professor Tony Volk from Brock University in Ontario, said: "We want to let parents know that if they're not instantly grabbed by this baby as much as they thought they might be, then that's normal.
"The bonding will build and grow over time."
Delayed baby attachment may have been an evolutionary advantage
The researchers were surprised by their findings, as they assumed younger babies would be thought of as the cutest.
They believed adults would feel the greatest urge to care for infants in the initial stages of their lives as this is when they are most vulnerable.
Yet, baby abandonment most commonly occurs during the first few weeks of infants' lives.
Speaking of the results, Professor Volk said: 'Hunter-gatherers who already had a child they were nursing, couldn't nurse two children at once.
'If you're a peasant mother in medieval England and you only have enough food for one child, and if having two means they're both likely to die, it's best just to have one child.
'These are difficult decisions that humans have made for thousands of years.
'A delay in attachment makes those early losses easier to cope with.'
Previous research also suggests babies develop a preference for a specific caregiver and experience 'separation anxiety' at around seven months old.
The researchers believe their findings suggest the need for society to encourage bonding between parents and their newborns via skin-to-skin contact and lots of interaction.
How the research was carried out
The researchers showed 142 people photographs of 18 babies taken shortly after birth, at three months old and then at six months.
They asked the study's participants how willing they would be to adopt these infants based on their cuteness, happiness, perceived health and self-resemblance.
The findings were published in the journal Evolution and Human Behavior.
Newborns who look like their fathers are healthy at one-year-old
This comes after research released earlier this month suggested newborns who look like their fathers are significantly healthier at one-year-old.
Children who resemble their fathers are less likely to have an asthma attack and visit the doctors or emergency room, as well as spending less time in hospital if they are admitted, a study found.
Fathers who do not live with their children spend around 2.5 more days a month with their offspring if they look like them, the research adds.
This is thought to be due to such fathers being more confident they are genetically related to their offspring, prompting them to invest greater attention in their youngsters' health needs and reducing their exposure to harm.
Study author Professor Solomon Polachek from Binghamton University, New York, said: "Fathers are important in raising a child and it manifests itself in the health of the child."