Tryptophan (TRP), the precursor of serotonin (5-HT), is one of the most investigated amino-acids and is contained in foods such as soy, almonds, spinach, etc. TRP supplementation can increase 5-HT levels in the brain and for this reason numerous studies have investigated whether administration of TRP can positively influence social behavior that relies on serotonergic function.

In earlier blogs, we already explained that TRP can increase interpersonal trust , increases the amount of money people donate to charity, may induce more sociable behavior in prisoners and may be a key factor in delevoping ways to support healthy aging.
In our recent publication, we review the available empirical studies on TRP, supplementation to clarify if and under what circumstances TRP supplementation might modulate social behavior. From our review, it seems that there is promising utility of TRP supplementation for patients or individuals suffering from disorders or behaviors related to dysfunctions in the 5-HT system, as TRP might help improve control over negative social behavior such as aggression, although studies on this issue are still limited. In contrast, in healthy humans TRP supplementation seems to promote (pro)social behavior such as agreeableness, sharing, helping, and donating. This suggests TRP supplementation may be a useful tool to enhance social functioning in inexpensive and efficient ways!

But how does it work?
TRP stimulates serotonin (5-HT) synthesis, which possibly induces a positive bias in information processing. This means that people allocate more attention to positive stimuli, and less attion to negative stimuli. This may result in more social behavior and less negative (e.g. aggressive) behavior. Interestingly, this already suggests the effect of TRP on social behaviors may be strongest for individuals with low baseline 5-HT functioning, as their initial bias towards negative stimuli might be greatest. However, evidence for this relation between TRP efficacy and initial 5-HT state is still controversial.

Aside from inducing a positive bias in information processing, the modulating effect of TRP on social behavior might also be mediated by other pathways. For example, TRP administration and increases in brain TRP are also associated with better quality of sleep and better mood, which might impact behavior in several ways. The relationship between TRP and quality of sleep is not surprising if one considers that 5-HT is also the precursor of melatonin, which plays an important role in regulating the sleep-wake cycle. Furthermore, TRP is reckoned to have a mild sedative effect, possibly due to the increase in melatonin production associated with the increase in 5-HT levels. Such a relation may explain, for example, the positive effects that TRP can have on impulsive behavior. The relation between TRP and mood may represent an alternative pathway through which TRP can affect social behavior. As pointed out by Young (2013), given that increases in 5-HT may have positive effects on mood, and as better mood is typically associated with more positive social interactions, the effects of TRP in promoting social behavior may just reflect the consequence of better mood following TRP intake, though the opposite may be true as well.

Interestingly, improved sleep and mood are related to reduced stress and better coping abilities. When experiencing stress, people tend to behave and process information in ways that are less resource demanding. As taking into account the mental states of others can be considered resource demanding, someone that is stressed may process information in a more egocentric or self-centered way (and as a consequence, to behave less prosocially; but the opposite might also be true).Therefore, one may argue that TRP may improve social behavior by reducing stress. However, to date this represents a question rather than a fact.

For detailed explanations of a number of possible pathways and factors modulating the effectivity of TRP, see the complete review.

The problem with measuring social behavior
We would like to point out that in laboratory studies such as the ones discussed in our review, social behavior is typically measured by attitudes, behavioral indicators of helping or self-reported intent to help, aggressiveness, etc. However, more direct behavioral measures such as charitable donating and aggressive incidents are sometimes used as well. Aside from the possibility that attitudes assess socially desirable responses and not behavior per se, the variability in measures used to assess social behavior makes the reviewed studies hard to compare. It would therefore be interesting to use direct measures of social behavior, as this may help gaining a better understanding on how TRP can affect social behavior in real-life situations, outside the lab. In addition, other measures of prosocial behavior that have not yet been investigated in relation to TRP, such as how much time people are willing to spend with others, could be considered as well.

Although more research is needed to disentangle and understand the relation between individual differences, TRP effectivity, 5-HT functioning, and social contexts and interactions, we conclude TRP can be an effective method of modulating social behavior.

Steenbergen, L., Jongkees, B. J., Sellaro, R., & Colzato, L. S. (2016). Tryptophan supplementation modulates social behavior: A review. Neuroscience & Biobehavioral Reviews, 64, 346-358.