Research has shown that heavier newborns have a reduced risk for lifetime depression and anxiety symptoms. Conversely, young children who are delayed in first walking or standing appear to have an increased risk for developing symptoms of depressive and anxiety related disorders.

These conclusions were reached by a team who examined nearly fifty years worth of records from the British based Medical Research Council National Survey of Health & Development. This survey included data on over 4,600 individuals beginning with the cohort's birth in 1946. Researchers divided survey responders into one of six groups depending on their lifetime mental health. They then examined statistical data regarding each group's early childhood experiences. Analyzing these statistical relationships, the researchers were able to identify the factors that appear to correlate with an increased prevalence of mental health concerns.

Previous research had linked low birth weight to depression in girls. Inquiry into birth weight has been much more varied, though, and studies have shown that everything from intelligence to sudden infant death syndrome may in fact be linked back to birth weight.

One way of thinking about birth weight is as a marker of development (a starting point for all future development). In this way, many of these potential outcomes begin to come into focus. Both depression and anxiety, if discussed purely as diseases of the mind, can be understood as markers of brains that have been taken off the course of healthy progression or have some anomaly in their development. Any number of factors can cause this to happen (think: alcohol, drug abuse or emotional trauma), and this current research has merely identified another possibility.

Birth weight is by no means a strong or reliable predictor, but this line of research seeks to highlight the fact that it may be part of the puzzle. Delayed standing and walking could themselves be stronger signs that development has undergone a hiccup. It could be that low birth weight does indeed correlate with an increased risk of future mental illness, but that it simply does not correlate strongly enough for it to be considered significant. In either case, future inquiry into the ways that birth weight affect future outcomes will illuminate more.