Children who start puberty by the age of eight or nine have poorer mental health, according to an Australian study.
But it's not the experiences of puberty that cause the adolescent angst — children who start puberty earlier show poor mental health signs from the age of four.
Researchers from the Murdoch Children's Research Institute found boys who go through early puberty have greater behavioural difficulties and poorer emotional and social adjustment from the age of four.
Girls who start puberty by eight or nine didn't have behavioural problems, but did show more social adjustment and emotional problems from when they were four.
The researchers studied almost 3500 children from the age of four until they were 11. Their parents were interviewed four times over the period and asked about signs of puberty, as well as behaviour, emotions and social functioning.
The authors took into account other risk factors for early puberty and poor mental health, such as body mass index, socioeconomic status and ethnicity.
"We think that the association between early onset puberty and poorer adolescent mental health is due to developmental processes that start well before the onset of puberty and continue into adolescence," said lead researcher Dr Fiona Mensah.
"There is a heightened risk for behaviour and emotional problems during puberty; and children who reach puberty earlier than their peers have more of these difficulties in adolescence."
Early puberty and mental health link
Past studies have looked at the link between early puberty and mental health, but this is the first study that looks at whether there were already mental health problems from earlier childhood.
"Children who go through puberty earlier tend to have more depression, anxiety, disruptive behaviour and substance abuse," Dr Mensah said.
"Traditionally these difficulties have been viewed as a consequence of the early pubertal transition … but we found there were pre-standing difficulties and more of a life-course risk."
Professor George Patton, who was involved in the study, said the starting age of puberty is decreasing at the same time as adolescent mental health conditions are increasing.
"We don't know why it might be happening, but these changes in the age of onset of puberty might be one reason why we see a greater vulnerability than we have in times gone by," he said.
However Professor Patton said early onset puberty is not the only thing to blame for mental health problems.
"It's not likely to be an explanation in itself because the social environments in which kids are growing up have changed profoundly and continues to change," he said.
"One of the big changes is the emergence of social networking and digital media … and there are significant emotional hazards such as cyber bullying and sleep problems that can affect kids in these years."
Why we're developing earlier
Scientists are still trying to determine why puberty is hitting earlier.
One of the main determinants for early puberty is better nutrition and weight during childhood. Kids who are heavier during childhood tend to go into puberty at an earlier point than those who are skinnier," Professor Patton said.
"There are genetic factors that are really important as well — if your mum or dad went into puberty at an early point, you are more likely to go into puberty early. There has been a whole question about whether there may be some influence of chemicals that are found in the environment known as endocrine disrupters that mimic some of the effects of hormones that might be involved in kickstarting puberty at an earlier point."
The study was published in the Journal of Adolescent Health.