Poverty in early childhood can shrink the brain, a study has shown.
The finding may help explain links between deprivation and poor mental ability and performance at school.
Scientists in the United States conducted magnetic resonance imaging (MRI) brain scans on 145 children aged six to 12.
They found that poverty was associated with smaller hippocampus and amygdala brain regions.
Both have roles in processing memory, and the amygdala is also linked to emotional reactions.
Amounts of grey and white matter were also lower in the brains of poor children.
Grey matter contains the cell bodies of neurons while white matter consists of the axons, or wiring, connecting different parts of the brain together.
The brain-shrinking effect of poverty was significantly more pronounced in children with stressful home environments whose parents lacked nurturing skills, the scans showed.
Child psychiatrist Professor Joan Luby, from Washington University, said: "We've known for many years from behavioral studies that exposure to poverty is one of the most powerful predictors of poor developmental outcomes for children.
"A growing number of neuroscience and brain-imaging studies recently have shown that poverty also has a negative effect on brain development.
"What's new is that our research shows the effects of poverty on the developing brain, particularly in the hippocampus, are strongly influenced by parenting and life stresses that the children experience."
The research is published in the latest online issue of the journal JAMA Pediatrics.
The children were recruited from a larger group taking part in a pre-school depression study.
As the children have grown, they have undergone MRI scans that track brain development.
" We actually stumbled upon this finding," said Prof Luby.
"Initially, we thought we would have to control for the effects of poverty, but as we attempted to control for it, we realised that poverty was really driving some of the outcomes of interest, and that caused us to change our focus to poverty, which was not the initial aim of this study."
Some of the children were depressed, others in good mental health, and a number diagnosed with psychiatric conditions such as attention-deficit hyperactivity disorder (ADHD).
Poverty was measured by the "income-to-needs ratio" which takes into account a family's size and annual income.
Currently in the US, the poverty threshold for a family of four is an income of 23,550 dollars (£14,532) per year.
A test for nurturing was devised that involved giving a child a gift-wrapped present which could only be opened after his or her parent completed the necessary paperwork, a task estimated to take 10 minutes.
Poorer nurturers became more stressed and impatient during the exercise.
"Parents can be less emotionally responsive for a whole host of reasons," said Prof Luby.
"They may work two jobs or regularly find themselves trying to scrounge together money for food. Perhaps they live in an unsafe environment. They may be facing many stresses, and some don't have the capacity to invest in supportive parenting as much as parents who don't have to live in the midst of those adverse circumstances."
In an accompanying editorial, US expert Dr Charles Nelson, from Harvard Medical School, wrote: "Early experience weaves its way into the neural and biological infrastructure of the child in such a way as to impact developmental trajectories and outcomes.
"Exposure to early life adversity should be considered no less toxic than exposure to lead, alcohol or cocaine."