Minocycline, a microglial inhibitor, reduces ‘honey trap’ risk in human economic exchange

Scientific Reports; 3,
Article number:: 1685; doi:10.1038/srep01685

Received: 17 December 2012
Accepted: 27 March 2013
Published: 18 April 2013

Recently, minocycline, a tetracycline antibiotic, has been reported to improve symptoms of psychiatric disorders and to facilitate sober decision-making in healthy human subjects. Here we show that minocycline also reduces the risk of the ‘honey trap’ during an economic exchange. Males tend to cooperate with physically attractive females without careful evaluation of their trustworthiness, resulting in betrayal by the female. In this experiment, healthy male participants made risky choices (whether or not to trust female partners, identified only by photograph, who had decided in advance to exploit the male participants). The results show that trusting behaviour in male participants significantly increased in relation to the perceived attractiveness of the female partner, but that attractiveness did not impact trusting behaviour in the minocycline group. Animal studies have shown that minocycline inhibits microglial activities. Therefore, this minocycline effect may shed new light on the unknown roles microglia play in human mental activities.

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At a glance

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Introduction

In movies, a female spy often wins the trust of her male target using her physical attractiveness. The male target usually suspects that she is a spy, but because of her attractiveness, he becomes amorously entangled with the female spy despite concerns regarding her trustworthiness. For males, allocating valuable resources to physically attractive females may be evolutionarily adaptive, in that it may increase the probability of producing attractive offspring under natural selection. However, this tendency toward resource allocation to attractive females creates ‘noise’ that complicates decisions in short-term economic exchanges, leading to the tendency to ‘honey trap’ males with this behaviour.

In an economic exchange, attractiveness in a female increases sexual arousal in a male that automatically (without careful evaluation of her trustworthiness) facilitates trusting behaviour. While these traits should be adaptive in terms of mate-choice¹, experimental studies have shown that they also affect decisions in social and economic exchange^2, 3. These traits lead to the question of how males can avoid the honey trap.

Recent studies with human subjects show that minocycline, a commonly used tetracycline antibiotic, may facilitate focus on appropriate environmental cues for social decision-making, possibly by reducing noise and other factors (e.g. personality and arousal) that can obstruct decisions. In an economic exchange, one study showed that subjects treated with minocycline make more sober decisions compared to participants treated with placebo⁴. In another study, participants were given dextroamphetamine and those treated with minocycline report less of a ‘high’ feeling compared to those who did not receive minocycline⁴. Minocycline is also known to improve symptoms associated with psychiatric disorders such as schizophrenia and depression^5, 6,7. There are past studies examining the effects of physical attractiveness on cooperation in social/economic exchange in different sex pairs, but no study has examined the effects of minocycline on such behaviour in different sex pairs. The hypothesis of this study was that minocycline reduces the risk of the honey trap effect and leads to more appropriate decisions in a short-term economic exchange, through a reduction in the noise triggered by physical attractiveness.

In this experiment, 98 healthy males played a trust game with 8 photographed young females after a 4-day oral treatment course of either minocycline or placebo. Looking at a picture showing a female's face, male players decided how much out of 1300 yen (approximately 13 USD) they would give to each female. Males then evaluated how trustworthy each female was and how physically attractive she was using a 11-point Likert Scale (0: Not at all – 10: Perfectly so). Of note, all of the photographed females had actually decided, in advance, to choose ‘betray’ against the male players. Therefore, male participants played with untrustworthy female partners, but were unaware of the deception. The impact of attractiveness and trustworthiness on the amount of money given to female partners was analysed. The independent variables were the evaluations/scores of physical attractiveness and trustworthiness given by the male participants.

Results

Table 1 summarizes the mean scores for the major variables and results of a t-test used to compare the placebo and minocycline conditions. Consistent with previous reports in which trust games were conducted between healthy male participants^8, 9, the offering rate differed marginally between conditions. The State and Trait Anxiety Inventory (STAI)¹⁰ was measured and no significant differences were found for either State or Trait Anxiety scores between conditions.

Table 1: Mean scores and results of t-tests comparing major variables

Full table

The primary hypothesis of this study was that the minocycline group would be less affected by the attractiveness of pictured females than the placebo group. To test this hypothesis, an ANOVA was performed with condition (minocycline vs. placebo) and attractiveness (high vs. low) as independent variables and the offering rate of money by participants as the dependent variable. The attractiveness score was not normally distributed (P = 0.0004), therefore the score was sub-divided into 2 categories (high vs. low). Figure 1 shows the mean offer rate by condition and the level of attractiveness. There is a significant interaction effect between condition and attractiveness (F (1,776) = 7.78, P = 0.005). Consistent with the primary hypothesis, participants in the placebo group gave larger amounts of money when the partner was more attractive, while participants in the minocycline group did not. According to a simple main effect test, a main effect of attractiveness was detected in the placebo group (P = 0.0004), but not in the minocycline group (P= 0.223). In addition, Figure 1 shows that, for partners with high attractiveness, the offering rate in the placebo group was significantly higher than in the minocycline group (P = 0.0004), but not for less attractive partners (P = 0.590).

Figure 1: Mean Offering Rate (percentage of money offered) by the Male Participants to Less- and More-Attractive Female Partners.

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